Semiconductor device, display device, and electronic device

ABSTRACT

A pixel includes a load, a transistor which controls a current supplied to the load, a storage capacitor, and first to fourth switches. By inputting a potential in accordance with a video signal into the pixel after the threshold voltage of the transistor is held in the storage capacitor, and holding a voltage of the sum of the threshold voltage and the potential, variations of a current value caused by variations of threshold voltage of a transistor can be suppressed. Consequently, a predetermined current can be supplied to the load such as a light-emitting element. Further, by changing the potential of a power supply line, a display device with a high duty ratio can be provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/514,535, filed Oct. 15, 2014, now allowed, which is a continuation ofU.S. application Ser. No. 14/072,850, filed Nov. 6, 2013, now U.S. Pat.No. 8,872,739, which is a continuation of U.S. application Ser. No.13/430,773, filed Mar. 27, 2012, now U.S. Pat. No. 8,599,115, which is acontinuation of U.S. application Ser. No. 13/175,998, filed Jul. 5,2011, now U.S. Pat. No. 8,164,547, which is a continuation of U.S.application Ser. No. 12/903,662, filed Oct. 13, 2010, now U.S. Pat. No.7,982,696, which is a continuation of U.S. application Ser. No.11/695,788, filed Apr. 3, 2007, now U.S. Pat. No. 7,817,117, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2006-104191 on Apr. 5, 2006, all of which are incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having afunction for controlling a current supplied to a load with a transistor,and relates to a display device including a pixel which is formed of acurrent-drive display element, luminance of which is changed inaccordance with a signal, and a signal line driver circuit or scan linedriver circuit which drives the pixel. In addition, the inventionrelates to a driving method of such a semiconductor device and a displaydevice. Further, the invention relates to an electronic device havingthe display device in a display portion.

2. Description of the Related Art

In recent years, a self-luminous display device having a pixel formed byusing a light-emitting element such as an electroluminescence (EL)element, that is, a so-called light-emitting device has attractedattention. As a light-emitting element which is used for such aself-luminous display device, an organic light-emitting diode (OLED) andan EL element have attracted attention, and they have been used for anEL display or the like. Since these light-emitting elements emit lightby themselves, an EL display or the like has advantages compared to aliquid crystal display such that it has higher pixel visibility, nobacklight is required, and response speed is higher. Note that luminanceof a light-emitting element is, in many cases, controlled by a currentvalue flowing to the light-emitting element.

In addition, an active matrix display device in which a transistor whichcontrols light emission of a light-emitting element is provided in eachpixel has been developed. The active matrix display device has beenexpected to be put into practical use because not only it can realizehigh definition and large-screen display which is difficult to realizein a passive matrix display device, but also it can operate with lesspower consumption than the passive matrix display device.

FIG. 50 shows a pixel configuration of a conventional active matrixdisplay device (Reference 1: Japanese Published Patent Application No.H08-234683). The pixel shown in FIG. 50 includes a thin film transistor(TFT) 11, a TFT 12, a capacitor 13, and a light-emitting element 14, andis connected to a signal line 15 and a scan line 16. Note that a powersupply potential Vdd is supplied to either a source electrode or a drainelectrode of the TFT 12 and one electrode of the capacitor 13, and aground potential is supplied to an opposite electrode of thelight-emitting element 14.

At this time, in the case of using amorphous silicon for a semiconductorlayer of the TFT 12 which controls a current value supplied to thelight-emitting element 14, that is, a driving TFT, the threshold voltage(Vth) fluctuates due to deterioration or the like. In that case,although the same potential is applied from the signal line 15 todifferent pixels, a current flowing to the light-emitting element 14 isdifferent in each pixel, and display luminance becomes ununiformdepending on the pixels. Note that also in the case of using polysiliconfor the semiconductor layer of the driving TFT, characteristics of thetransistor deteriorate or vary.

In order to overcome this problem, an operating method using a pixel inFIG. 51 is proposed in Reference 2 (Reference 2: Japanese PublishedPatent Application No. 2004-295131). The pixel shown in FIG. 51 includesa transistor 21, a driving transistor 22 which controls a current valuesupplied to a light-emitting element 24, a capacitor 23, and thelight-emitting element 24, and is connected to a signal line 25 and ascan line 26. Note that the driving transistor 22 is an NMOS transistor,and a ground potential is supplied to either a source electrode or adrain electrode of the driving transistor 22 and Vca is supplied to anopposite electrode of the light-emitting element 24.

FIG. 52 shows a timing chart of an operation of this pixel. In FIG. 52,one frame period is divided into an initialization period 31, athreshold voltage (Vth) writing period 32, a data writing period 33, anda light-emitting period 34. Note that one frame period corresponds to aperiod for displaying an image for one screen, and the initializationperiod, the threshold voltage (Vth) writing period, and the data writingperiod are collectively described as an address period.

First, the threshold voltage of the driving transistor 22 is writteninto the capacitor 23 in the threshold voltage writing period 32. Afterthat, a data voltage (Vdata) showing luminance of the pixel is writteninto the capacitor 23 and Vdata+Vth is stored in the capacitor 23 in thedata writing period 33. Then, the driving transistor 22 is turned on inthe light-emitting period 34, so that the light-emitting element 24emits light with luminance specified by the data voltage by changingVca. By performing such an operation, variations in luminance caused byfluctuations of the threshold voltage of the driving transistor 22 arereduced.

Reference 3 (Reference 3: Japanese Published Patent Application No.2004-280059) also discloses that a voltage of the sum of the thresholdvoltage of a driving TFT and a data potential corresponds to agate-source voltage of the driving TFT, so that a current flowing to alight-emitting element does not change even when the threshold voltageof the TFT fluctuates.

SUMMARY OF THE INVENTION

As described above, in a display device, suppression of variations of acurrent value caused by variations in the threshold voltage of a drivingTFT has been expected.

In each of the operating methods disclosed in Reference 2 and Reference3, the initialization, the threshold voltage writing, and the lightemission are performed by changing a potential of Vca several times ineach one frame period. In each pixel disclosed in Reference 2 andReference 3, since one electrode of a light-emitting element to whichVca is supplied, that is, an opposite electrode thereof is formed overthe entire pixel region, the light-emitting element cannot emit light ifthere is even one pixel in which a data writing operation is performedother than the initialization and the threshold voltage writing.Therefore, as shown in FIG. 53, a ratio of a light-emitting period inone frame period (i.e., a duty ratio) becomes low.

When the duty ratio is low, the amount of a current supplied to alight-emitting element and a driving transistor is required to beincreased, so that a voltage applied to the light-emitting elementbecomes higher and power consumption also becomes higher. In addition,since the light-emitting element and the driving transistor easilydeteriorate, screen burn-in is generated or higher power is required inorder to obtain luminance which is almost equal to luminance beforedeterioration.

In addition, since the opposite electrode is connected to all of thepixels, the light-emitting element functions as an element having largecapacitance. Therefore, in order to change a potential of the oppositeelectrode, high power consumption is required.

In view of the foregoing problems, it is an object of the invention toprovide a display device with low power consumption and high brightness.It is another object of the invention to obtain a pixel configuration, asemiconductor device, and a display device in which a deviation fromluminance specified by a data potential is small. Note that a target ofthe invention is not limited to only a display device having alight-emitting element, and it is another object of the invention tosuppress variations of a current value caused by variations in thethreshold voltage of a transistor.

A display device of the invention is provided with a pixel configurationin which a capacitance portion which can hold a potential of the sum ofa potential corresponding to the threshold voltage of a transistor and apotential in accordance with a video signal inputted to the transistoris provided between a gate and a source of the transistor which controlsa current supplied to a load (a display medium such as a light-emittingelement) which is controlled by the current. By holding the potential ofthe sum of the potential corresponding to the threshold voltage of thetransistor and the potential in accordance with the video signal in thecapacitance portion, current fluctuation caused by characteristicvariations of a current controlling transistor, that is, distortion ofimage quality can be suppressed. Note that the current is supplied bychanging a potential of a drain of the transistor.

In addition, in the case of inputting the potential in accordance withthe video signal in the pixel (a writing period), the transistor is madeto be turned off or a current path is made to be interrupted, so thatvoltage fluctuation of a capacitor caused by a current supplied from thetransistor can be suppressed.

Although the display device of the invention includes the transistorwhich controls a current and the load to which the current controlled bythe transistor is supplied, the load is not limited to a light-emittingelement typified by an electroluminescence (EL) element (an organic ELelement, an inorganic EL element, or an EL element including both anorganic material and an inorganic material); thus, a display medium,brightness, a color tone, polarized light, or the like of which ischanged by supplying a current therethrough can be applied to the load.

A semiconductor device in accordance with one aspect of the inventionincludes a pixel having a transistor, a first switch, a second switch,and a third switch. One of a source electrode and a drain electrode ofthe transistor is electrically connected to a pixel electrode and to afirst wiring through the second switch; the other of the sourceelectrode and the drain electrode of the transistor is electricallyconnected to a second wiring through the third switch; and a gateelectrode of the transistor is electrically connected to the secondwiring through the first switch. A signal in accordance with a grayscale is inputted to the gate electrode.

A semiconductor device in accordance with one aspect of the inventionincludes a transistor, a storage capacitor, a first switch, a secondswitch, a third switch, and a fourth switch. One of a source electrodeand a drain electrode of the transistor is electrically connected to apixel electrode and to a second wiring through the third switch; theother of the source electrode and the drain electrode of the transistoris electrically connected to a first wiring; and a gate electrode of thetransistor is electrically connected to the first wiring through thefourth switch and the second switch, to a third wiring through thefourth switch and the first switch, and to one of the source electrodeand the drain electrode of the transistor through the fourth switch andthe storage capacitor.

A semiconductor device in accordance with one aspect of the inventionincludes a transistor, a storage capacitor, a first switch, a secondswitch, a third switch, and a fourth switch. One of a source electrodeand a drain electrode of the transistor is electrically connected to apixel electrode and to a second wiring through the third switch; theother of the source electrode and the drain electrode of the transistoris electrically connected to a first wiring; and a gate electrode of thetransistor is electrically connected to the first wiring through thesecond switch, to a third wiring through the fourth switch and the firstswitch, and to one of the source electrode and the drain electrode ofthe transistor through the fourth switch and the storage capacitor.

A semiconductor device in accordance with one aspect of the inventionincludes a transistor, a storage capacitor, a first switch, a secondswitch, a third switch, and a fourth switch. One of a source electrodeand a drain electrode of the transistor is electrically connected to apixel electrode and to a second wiring through the third switch; theother of the source electrode and the drain electrode of the transistoris electrically connected to a first wiring through the fourth switch;and a gate electrode of the transistor is electrically connected to thefirst wiring through the second switch, to a third wiring through thefirst switch, and to one of the source electrode and the drain electrodeof the transistor through the storage capacitor.

A semiconductor device in accordance with one aspect of the inventionincludes a transistor, a storage capacitor, a first switch, a secondswitch, a third switch, and a fourth switch. One of a source electrodeand a drain electrode of the transistor is electrically connected to apixel electrode through the fourth switch and to a second wiring throughthe fourth switch and the third switch; the other of the sourceelectrode and the drain electrode of the transistor is electricallyconnected to a first wiring; and a gate electrode of the transistor iselectrically connected to the first wiring through the second switch, toa third wiring through the first switch, and to one of the sourceelectrode and the drain electrode of the transistor through the storagecapacitor and the fourth switch.

The second wiring may be the same as a wiring which controls the thirdswitch.

The second wiring may be any one of scan lines which control the firstto fourth switches of a previous row and the following row.

The transistor may be an N-channel transistor. In addition, asemiconductor layer of the transistor may be formed of a non-crystallinesemiconductor film. Further, the semiconductor layer of the transistormay also be formed of amorphous silicon.

Alternatively, the semiconductor layer of the transistor may be acrystalline semiconductor film.

In the aforementioned invention, a potential inputted to the firstwiring may be a binary value of V1 or V2; the potential inputted to thefirst wiring may be the value of V2 only when the first to thirdswitches are turned off and the fourth switch is turned on; thepotential of V1 may be a potential higher than a potential inputted tothe second wiring; the difference between the potential of V1 and thepotential inputted to the second wiring may be larger than the thresholdvoltage of the transistor; and the value of V2 may be larger than thevalue of V1.

In addition, the transistor may be a P-channel transistor. In that case,in the aforementioned invention, a potential inputted to the firstwiring may be a binary value of V1 or V2; the potential inputted to thefirst wiring may be the value of V2 only when the first to thirdswitches are turned off and the fourth switch is turned on; thepotential of V1 may be higher than a potential inputted to the secondwiring; the difference between the potential of V1 and the potentialinputted to the second wiring may be smaller than the absolute value ofthe threshold voltage of the transistor; and the value of V2 may besmaller than the value of V1.

A semiconductor device in accordance with one aspect of the inventionincludes a transistor, one of a source electrode and a drain electrodeof which is electrically connected to a first wiring and the other ofthe source electrode and the drain electrode of which is electricallyconnected to a second wiring; a storage capacitor which holds agate-source voltage of the transistor; a means which makes the storagecapacitor hold a first voltage by applying a first potential inputted tothe first wiring to one of electrodes of the storage capacitor andapplying a second potential inputted to the second wiring to the otherof the electrodes of the storage capacitor; a means which discharges avoltage of the storage capacitor down to a second voltage; a means whichmakes the storage capacitor hold a fifth voltage which is the sum of thesecond voltage and a fourth voltage by applying a potential which is thesum of the first potential and a third voltage to the one of theelectrodes of the storage capacitor; and a means which supplies acurrent set for the transistor to a load by inputting a third potentialwhich is different from the first potential to the first wiring.

A semiconductor device in accordance with one aspect of the inventionincludes a transistor, one of a source electrode and a drain electrodeof which is electrically connected to a first wiring and the other ofthe source electrode and the drain electrode of which is electricallyconnected to a second wiring; a storage capacitor which holds agate-source voltage of the transistor; a means which makes the storagecapacitor hold a first voltage by applying a first potential inputted tothe first wiring to one of electrodes of the storage capacitor andapplying a second potential inputted to the second wiring to the otherof the electrodes of the storage capacitor; a means which discharges avoltage of the storage capacitor down to the threshold voltage of thetransistor; a means which makes the storage capacitor hold a fourthvoltage which is the sum of the threshold voltage of the transistor anda third voltage by applying a potential which is the sum of the firstpotential and a second voltage to the one of the electrodes of thestorage capacitor; and a means which supplies a current set for thetransistor to a load by inputting a third potential which is differentfrom the first potential to the first wiring.

The transistor may be an N-channel transistor. In addition, asemiconductor layer of the transistor may be formed of a non-crystallinesemiconductor film. Further, the semiconductor layer of the transistormay also be formed of amorphous silicon.

Alternatively, the semiconductor layer of the transistor may be acrystalline semiconductor film.

In the aforementioned invention, the first potential may be higher thanthe second potential; the difference between the first potential and thesecond potential may be larger than the threshold voltage of thetransistor; and the first potential may be lower than the thirdpotential.

In addition, the transistor may be a P-channel transistor. In this case,the first potential may be lower than the second potential; thedifference between the first potential and the second potential may belarger than the absolute value of the threshold voltage of thetransistor; and the first potential may be higher than that of the thirdpotential.

Further, a display device including the aforementioned semiconductordevice and an electronic device including the display device in adisplay portion are included in accordance with one aspect of theinvention.

Note that various types of switches can be used as a switch described inthe specification, and an electrical switch, a mechanical switch, or thelike is given as an example. That is, any element can be used as long asit can control a current flow, and thus, a switch is not limited to acertain element. For example, it may be a transistor, a diode (e.g., aPN diode, a PIN diode, a Schottky diode, or a diode-connectedtransistor), or a logic circuit combining such elements. In the case ofusing a transistor as a switch, the polarity (a conductivity type) ofthe transistor is not particularly limited to a certain type because itoperates just as a switch. However, a transistor of polarity withsmaller off-current is preferably used. A transistor provided with anLDD region, a transistor with a multi-gate structure, or the like isgiven as an example of a transistor with smaller off-current. Inaddition, it is preferable that an N-channel transistor be used when apotential of a source electrode of the transistor which is operated as aswitch is closer to a low-potential-side power supply (e.g., Vss, GND,or 0 V), while a P-channel transistor is used when the potential of thesource electrode is closer to a high-potential-side power supply (e.g.,Vdd). This is because the absolute value of a gate-source voltage of thetransistor can be increased, so that the transistor can easily functionas a switch. Note that a CMOS switch may also be employed by using bothN-channel and P-channel transistors. By employing the CMOS switch, anoutput voltage is easily controlled with respect to various inputvoltages, so that the switch can be operated appropriately.

Note that in the invention, description “being connected” is synonymouswith description “being electrically connected”. Accordingly, in thestructures disclosed in the invention, another element which enables anelectrical connection (e.g., a switch, a transistor, a capacitor, aninductor, a resistor, or a diode) may be interposed between elementshaving a predetermined connection relation. Needless to say, theelements may be arranged without interposing another elementtherebetween, and description “being electrically connected” includesthe case where elements are directly connected.

Note that the load is not limited to a light-emitting element typifiedby an electroluminescence (EL) element as described above, and thus, adisplay medium, brightness, a color tone, polarized light, or the likeof which is changed by supplying a current therethrough can be appliedto the load. As such a display medium, a display medium, contrast ofwhich changes by an electromagnetic action, such as an electron-emissiveelement, a liquid crystal element, electronic ink, a grating light valve(GLV), a plasma display panel (PDP), a digital micromirror device (DMD),or the like can be employed, for example. In addition, a carbon nanotubecan also be used for the electron-emissive element. Note that displaydevices using EL elements include an EL display; display devices usingelectron-emissive elements include a field emission display (FED), anSED-type flat panel display (SED: Surface-conduction Electron-emitterDisplay), or the like; display devices using liquid crystal elementsinclude a liquid crystal display, a transmissive liquid crystal display,a semi-transmissive liquid crystal display, and a reflective liquidcrystal display; and display devices using electronic ink includeelectronic paper.

Note that a transistor is an element having at least three terminals ofa gate electrode, a drain region, and a source region, and has a channelforming region between the drain region and the source region. Here,since the source region and the drain region of the transistor changedepending on the structure, the operating condition, or the like of thetransistor, it is difficult to accurately define a range of the sourceregion or the drain region. Therefore, when a connection relation of thetransistor is described, one of electrodes connected to two terminals ofthe drain region and the source region is described as a first electrodeand the other electrode is described as a second electrode.

Note that in the invention, various types of transistors can be appliedto a transistor without limiting to a certain type. Accordingly, a thinfilm transistor (TFT) using a non-single crystalline semiconductor filmtypified by amorphous silicon or polycrystalline silicon, a transistorformed by using a semiconductor substrate or an SOI substrate, a MOStransistor, a junction transistor, a bipolar transistor, a transistorusing a compound semiconductor such as ZnO or a-InGaZnO, a transistorusing an organic semiconductor or a carbon nanotube, or othertransistors can be applied. In addition, various types of substrates canbe used as a substrate over which a transistor is formed withoutlimiting to a certain type. For example, the transistor can be formedover a single crystalline substrate, an SOI substrate, a glasssubstrate, a plastic substrate, a paper substrate, a cellophanesubstrate, a quartz substrate, a stone substrate, a stainless steelsubstrate, a substrate having stainless steel foil, or the like. Inaddition, a transistor may be formed over one substrate, and then, thetransistor may be transferred to another substrate.

Note that, as described above, the transistor in the invention may beany type of transistor and may be formed over any type of substrate.Accordingly, all of circuits may be formed over a glass substrate, aplastic substrate, a single crystalline substrate, an SOI substrate, orany other substrates. By forming all of the circuits over the samesubstrate, the number of component parts can be reduced to cut cost, orthe number of connections to the circuit components can be reduced toimprove reliability. Alternatively, a part of the circuits may be formedover one substrate and another part of the circuits may be formed overanother substrate. That is, not all of the circuits are required to beformed over the same substrate. For example, a part of the circuits maybe formed with transistors over a glass substrate and another part ofthe circuits may be formed over a single crystalline substrate or thelike, so that an IC chip thereof is provided over the glass substrate byCOG (Chip On Glass). Alternatively, the IC chip may be connected to theglass substrate by TAB (Tape Automated Bonding) or a printed wiringboard. In this manner, by forming a part of the circuits over the samesubstrate, the number of component parts can be reduced to cut cost, orthe number of connections to the circuit components can be reduced toimprove reliability. In addition, by forming a portion with a highdriving voltage or a portion with high driving frequency, which consumeslarge power, over another substrate, increase of power consumption canbe prevented.

A structure of a transistor can be various modes without limiting to acertain structure. For example, a multi-gate structure having two ormore gate electrodes may be used. By using the multi-gate structure,off-current can be reduced and the withstand voltage of the transistorcan be increased to improve reliability, or fluctuation of adrain-source current caused by fluctuation of a drain-source voltage canbe reduced when the transistor operates in a saturation region. Inaddition, a structure where gate electrodes are formed above and below achannel may be used. By using the structure where gate electrodes areformed above and below the channel, a channel region is enlarged toincrease the amount of a current flowing therethrough, or a depletionlayer can be easily formed to decrease the S value. In addition, astructure where a gate electrode is formed above a channel, a structurewhere gate electrodes are formed below a channel, a staggered structure,an inversely staggered structure, or a structure where a channel regionis divided into a plurality of regions, and the divided regions areconnected in parallel or in series may be used. A source electrode or adrain electrode may overlap with a channel (or a part of it). By usingthe structure where the source electrode or the drain electrode mayoverlap with the channel (or a part of it), the case where an electriccharge is accumulated in the part of the channel so that an operationbecomes unstable can be prevented. In addition, an LDD (Lightly DopedDrain) region may be provided. By providing the LDD region, off-currentcan be reduced and the withstand voltage of the transistor can beincreased to improve reliability, or characteristics in which adrain-source current does not fluctuate very much can be provided evenif a drain-source voltage fluctuates when the transistor operates in asaturation region.

Note also that one pixel corresponds to one element which can controlbrightness in the invention. For example, one pixel corresponds to onecolor element and brightness is expressed with the color element.Accordingly, in the case of a color display device having color elementsof R (Red), G (Green), and B (Blue), a minimum unit of an image isformed of three pixels of an R pixel, a G pixel, and a B pixel. Notethat the color elements are not limited to three colors, and colorelements with more than three colors may be employed. RGBW (W meanswhite), or RGB plus yellow, cyan, and/or magenta is given as an example.Alternatively, as another example, in the case of controlling brightnessof a color element by using a plurality of regions, one regioncorresponds to one pixel. For example, in the case of performing areagray scale display, a plurality of regions which controls brightness areprovided in each color element and gray scales are expressed with thewhole regions. In this case, one region which controls brightnesscorresponds to one pixel. In that case, one color element is composed ofa plurality of pixels, and regions which contribute to display may bedifferent depending on pixels. In addition, in the plurality of pixelswhich form one color element, the viewing angle may be widened byslightly varying signals supplied to the plurality of pixels.

Note that in this specification, a semiconductor device means a devicehaving a circuit including a semiconductor element (e.g., a transistoror a diode). The semiconductor device may also include all devices thatcan function by utilizing semiconductor characteristics. In addition, adisplay device includes not only a display panel itself where aplurality of pixels including a load are formed over the same substrateas a peripheral driver circuit which drives the pixels, but also adisplay panel attached with a flexible printed circuit (FPC) or aprinted wiring board (PWB).

In the invention, description that an object is “formed on” or “formedover” another object does not necessarily mean that the object is indirect contact with another object. The description includes the casewhere two objects are not in direct contact with each other, that is,the case where another object is sandwiched therebetween. Accordingly,for example, when it is described that a layer B is formed on (or over)a layer A, it includes both of the case where the layer B is formed indirect contact with the layer A, and the case where another layer (e.g.,a layer C or a layer D) is formed in direct contact with the layer A andthe layer B is formed in direct contact with the layer C or D.Similarly, when it is described that an object is formed above anotherobject, it does not necessarily mean that the object is in directcontact with another object, and another object may be sandwichedtherebetween. Accordingly, for example, when it is described that alayer B is formed above a layer A, it includes both of the case wherethe layer B is formed in direct contact with the layer A, and the casewhere another layer (e.g., a layer C or a layer D) is formed in directcontact with the layer A and the layer B is formed in direct contactwith the layer C or D. Similarly, when it is described that an object isformed below or under another object, it includes both of the case wherethe objects are in direct contact with each other, and the case wherethe objects are not in contact with each other.

By employing the invention, variations of the current value caused byvariations in the threshold voltage of a transistor can be suppressed.Therefore, a desired current can be supplied to a load such as alight-emitting element. In particular, in the case of using alight-emitting element as a load, a display device with few variationsin luminance and a high ratio of a light-emitting period in one frameperiod can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram showing a pixel configuration shown in EmbodimentMode 1;

FIG. 2 is a timing chart showing an operation of the pixel shown in FIG.1;

FIGS. 3A to 3D are diagrams each showing an operation of the pixel shownin FIG. 1;

FIG. 4 is a model diagram of voltage-current characteristics inaccordance with channel length modulation;

FIG. 5 is a diagram showing a pixel configuration shown in EmbodimentMode 1;

FIG. 6 is a diagram showing a pixel configuration shown in EmbodimentMode 1;

FIG. 7 is a diagram showing a pixel configuration shown in EmbodimentMode 1;

FIG. 8 is a diagram showing a pixel configuration shown in EmbodimentMode 1;

FIG. 9 is a diagram showing a display device shown in Embodiment Mode 1;

FIG. 10 is a chart showing a writing operation of a display device shownin Embodiment Mode 1;

FIGS. 11A to 11F are diagrams each showing a pixel configuration shownin Embodiment Mode 2;

FIG. 12 is a diagram showing a pixel configuration shown in EmbodimentMode 3;

FIG. 13 is a diagram showing a pixel configuration shown in EmbodimentMode 3;

FIG. 14 is a diagram showing a pixel configuration shown in EmbodimentMode 3;

FIG. 15 is a diagram showing a pixel configuration shown in EmbodimentMode 3;

FIG. 16 is a diagram showing a pixel configuration shown in EmbodimentMode 1;

FIG. 17 is a partial sectional view showing a pixel shown in EmbodimentMode 7;

FIGS. 18A and 18B are diagrams each showing a light-emitting elementshown in Embodiment Mode 7;

FIGS. 19A to 19C are diagrams each showing an extraction direction oflight shown in Embodiment Mode 7;

FIGS. 20A and 20B are partial sectional views each showing a pixel shownin Embodiment Mode 7;

FIGS. 21A and 21B are partial sectional views each showing a pixel shownin Embodiment Mode 7;

FIGS. 22A and 22B are partial sectional views each showing a pixel shownin Embodiment Mode 7;

FIG. 23 is a partial sectional view showing a pixel shown in EmbodimentMode 7;

FIG. 24 is a partial sectional view showing a pixel shown in EmbodimentMode 7;

FIGS. 25A and 25B are diagrams showing a display device shown inEmbodiment Mode 9;

FIGS. 26A and 26B are diagrams each showing a display device shown inEmbodiment Mode 9;

FIGS. 27A and 27B are diagrams each showing a display device shown inEmbodiment Mode 9;

FIG. 28 is a partial sectional view showing a pixel shown in EmbodimentMode 9;

FIG. 29 is a diagram showing a pixel configuration shown in EmbodimentMode 4;

FIG. 30 is a diagram showing a pixel configuration shown in EmbodimentMode 4;

FIG. 31 is a diagram showing a pixel configuration shown in EmbodimentMode 5;

FIG. 32 is a timing chart showing operations of the pixel shown in FIG.31;

FIGS. 33A to 33H are diagrams showing electronic devices to which theinvention can be applied;

FIG. 34 is a view showing a structural example of a mobile phone;

FIG. 35 is a view showing an example of an EL module;

FIG. 36 is a block diagram showing a main configuration of an ELtelevision receiver;

FIG. 37 is a diagram showing a pixel configuration shown in EmbodimentMode 5;

FIG. 38 is a top plan view of the pixel shown in FIG. 5;

FIG. 39 is a diagram showing a pixel configuration shown in EmbodimentMode 6;

FIG. 40 is a timing chart showing operations of the pixel shown in FIG.39;

FIGS. 41A to 41D are diagrams each showing an operation of the pixelshown in FIG. 39;

FIG. 42 is a diagram showing a pixel configuration shown in EmbodimentMode 5;

FIG. 43 is a chart showing a driving method in which a digital grayscale method and a time gray scale method are combined;

FIGS. 44A to 44D are diagrams each showing an operation of the pixelshown in Embodiment Mode 1;

FIG. 45 is a diagram showing a pixel configuration shown in EmbodimentMode 1;

FIGS. 46A to 46C are diagrams each showing a light-emitting elementshown in Embodiment Mode 8;

FIGS. 47A to 47C are diagrams each showing a light-emitting elementshown in Embodiment Mode 8;

FIG. 48 is a diagram showing a pixel configuration shown in EmbodimentMode 1;

FIG. 49 is a diagram showing a pixel configuration shown in EmbodimentMode 6;

FIG. 50 is a diagram showing a pixel configuration of a conventionaltechnique;

FIG. 51 is a diagram showing a pixel configuration of a conventionaltechnique;

FIG. 52 is a timing chart for operating the pixel shown in aconventional technique; and

FIG. 53 is a chart showing a ratio of a light-emitting period in oneframe period in the case of using a conventional technique.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, modes of the invention is described. However, the inventioncan be implemented with various different modes and it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Unless such changes and modifications departfrom the spirit and the scope of the invention, they should be construedas being included therein. Therefore, the invention should not beconstrued as being limited to the description of the modes. Note that instructures of the invention described below, reference numerals showingthe same portions are used in common among the different drawings.

Embodiment Mode 1

A basic configuration of a pixel of the invention is described withreference to FIG. 1. A pixel shown in FIG. 1 includes a transistor 110,a first switch 111, a second switch 112, a third switch 113, a fourthswitch 114, a capacitor 115, and a light-emitting element 116. Note thatthe pixel is connected to a signal line 117, a first scan line 118, asecond scan line 119, a third scan line 120, a fourth scan line 121, apower supply line 122, and a potential supply line 123. In thisembodiment mode, the transistor 110 is an N-channel transistor, and isturned on when a gate-source voltage (Vgs) thereof exceeds the thresholdvoltage (Vth). In addition, an example is described in which an ELelement in which a current is supplied from a pixel electrode 4811 to anopposite electrode 124 is used as the light-emitting element 116 asshown in FIG. 48. In that case, the pixel electrode 4811 of thelight-emitting element 116 functions as an anode and the oppositeelectrode 124 thereof functions as a cathode. Note that a gate-sourcevoltage of the transistor is described as Vgs; a drain-source voltage ofthe transistor is described as Vds; the threshold voltage of thetransistor is described as Vth; and a voltage stored in the capacitor isdescribed as Vcs. The power supply line 122, the potential supply line123, and the signal line 117 are also described as a first wiring, asecond wiring, and a third wiring, respectively. Further, the first scanline 118, the second scan line 119, the third scan line 120, and thefourth scan line 121 may be described as a fourth wiring, a fifthwiring, a sixth wiring, and a seventh wiring, respectively.

A first electrode (one of a source electrode and a drain electrode) ofthe transistor 110 is connected to the pixel electrode of thelight-emitting element 116; a second electrode (the other of the sourceelectrode and the drain electrode) of the transistor 110 is connected tothe power supply line 122; and a gate electrode of the transistor 110 isconnected to the power supply line 122 through the fourth switch 114 andthe second switch 112. Note that the fourth switch 114 is connectedbetween the gate electrode of the transistor 110 and the second switch112. In addition, if a connection point of the fourth switch 114 and thesecond switch 112 is denoted by a node 130, the node 130 is connected tothe signal line 117 through the first switch 111. Further, the firstelectrode of the transistor 110 is also connected to the potentialsupply line 123 through the third switch 113.

In addition, the capacitor 115 is connected between the node 130 and thefirst electrode of the transistor 110. That is, a first electrode of thecapacitor 115 is connected to the gate electrode of the transistor 110through the fourth switch 114, and a second electrode of the capacitor115 is connected to the first electrode of the transistor 110. Thecapacitor 115 may be formed by sandwiching an insulating film with awiring, a semiconductor layer, or an electrode, or can be omitted byusing gate capacitance of the transistor in some cases. Such a meanswhich holds a voltage is described as a storage capacitor. Further, aconnection point of the node 130 and a wiring to which the first switch111 and the first electrode of the capacitor 115 are connected isdenoted by a node 131, and a connection point of the first electrode ofthe transistor 110 and a wiring to which the second electrode of thecapacitor 115 and the pixel electrode of the light-emitting element 116are connected is denoted by a node 132.

By inputting signals into the first scan line 118, the second scan line119, the third scan line 120, and the fourth scan line 121, on/off ofthe first switch 111, the second switch 112, the third switch 113, andthe fourth switch 114 is controlled, respectively.

A signal in accordance with a gray scale of the pixel which correspondsto a video signal, that is, a potential in accordance with luminancedata is inputted to the signal line 117.

Next, operations of the pixel shown in FIG. 1 are described withreference to a timing chart in FIG. 2, and FIGS. 3A to 3D. Note that, inFIG. 2, one frame period which corresponds to a period for displaying animage for one screen is divided into an initialization period, athreshold voltage (Vth) writing period, a data writing period, and alight-emitting period. In addition, the initialization period, thethreshold voltage (Vth) writing period, and the data writing period arecollectively described as an address period. Although one frame periodis not particularly limited to a certain period, it is preferable thatone frame period be at least 1/60 second or less so that an image viewerdoes not perceive a flicker.

Note that a potential of V1 is inputted to an opposite electrode 124 ofthe light-emitting element 116 and a potential of V1−Vth−α (α: anarbitrary positive number) is inputted to the potential supply line 123.In addition, the potential of V1 is inputted to the power supply line122 in the address period, and a potential of V2 is inputted to thepower supply line 122 in the light-emitting period. Note that V2>V1 issatisfied. That is, the potentials of the power supply line 122 and thepotential supply line 123 in the initialization period may be anypotential as long as a potential difference between the potentials ofthe power supply line 122 and the potential supply line 123 is apotential which turns on the transistor 110.

Here, although a potential of the opposite electrode 124 of thelight-emitting element 116 is the same as a potential of the powersupply line 122 in the address period for description of the operations,the potential of the opposite electrode 124 may be any potential as longas it is higher than a potential of V1−Vth−α−V_(EL) when a potentialdifference which is at least necessary for the light-emitting element116 to emit light is V_(EL). That is, in the address period, potentialsof both ends of the light-emitting element 116 may be any potential aslong as a current does not flow to the light-emitting element 116. Inaddition, the potential V2 of the power supply line 122 in thelight-emitting period may be any potential as long as it is higher thanthe sum of the potential of the opposite electrode 124 and the potentialdifference (V_(EL)) which is at least necessary for the light-emittingelement 116 to emit light; here, since the potential of the oppositeelectrode 124 is V1 for description, V2 may be any potential higher thanV1+V_(EL).

First, in the initialization period, the first switch 111 is turned offand the second switch 112, the third switch 113, and the fourth switch114 are turned on as shown in FIGS. 2A and 3A. At this time, the firstelectrode of the transistor 110 serves as the source electrode, and apotential thereof is equal to a potential of the potential supply line123 which is V1−Vth−α. On the other hand, a potential of the gateelectrode of the transistor 110 is V1. Therefore, a gate-source voltageVgs of the transistor 110 is Vth+α so that the transistor 110 is turnedon. Then, Vth+α is held in the capacitor 115 provided between the gateelectrode and the first electrode of the transistor 110. Although thecase where the fourth switch 114 is turned on is described, the fourthswitch 114 may be turned off as long as the capacitor 115 can hold avoltage which turns on the transistor 110. Note that in the followingthreshold voltage writing period, the fourth switch 114 is required tobe turned on.

A threshold voltage writing period (B) shown in FIG. 2, and in FIG. 3B,the third switch 113 is turned off. Therefore, the potential of thefirst electrode, that is, the source electrode of the transistor 110rises gradually, and when the potential reaches V1−Vth, that is, whenthe gate-source voltage Vgs of the transistor 110 reaches the thresholdvoltage (Vth), the transistor 110 is turned off. Accordingly, a voltageheld in the capacitor 115 is approximately Vth.

In the following data writing period (C) shown in FIG. 2, and in FIG.3C, the first switch 111 is turned on after the second switch 112 andthe fourth switch 114 are turned off, and a potential in accordance withluminance data (V1+Vdata) is inputted from the signal line 117. Byturning off the fourth switch 114 in this period, the transistor 110 canbe held to be turned off. Therefore, potential fluctuation of the secondelectrode of the capacitor 115 caused by a current supplied from thepower supply line 122 at the time of data writing can be suppressed.Accordingly, a voltage Vcs which is held in the capacitor 115 at thistime can be represented by Formula (1) when electrostatic capacitance ofthe capacitor 115 is C1 and electrostatic capacitance of thelight-emitting element 116 is C2.

$\begin{matrix}\lbrack {{Formula}\mspace{14mu} 1} \rbrack & \; \\{\;{{Vcs} = {{Vth} + {{Vdata} \times \frac{C\; 2}{{C\; 1} + {C\; 2}}}}}} & (1)\end{matrix}$

Note that since the light-emitting element 116 has thinner filmthickness and a larger electrode area than the capacitor 115, C2>>C1 issatisfied. Therefore, the voltage Vcs which is held in the capacitor 115is represented by Formula (2) from C2/(C1+C2)≈1. Note also that in thecase where the light-emitting element 116 is controlled not to emitlight in the following light-emitting period, a potential V1+Vdata(Vdata≦0) is input.[Formula 2]Vcs=Vth+Vdata  (2)

Next, in the light-emitting period (D) shown in FIG. 2, and in FIG. 3D,the first switch 111 is turned off, and the fourth switch 114 is turnedon after the potential of the power supply line 122 is made V2. At thistime, the gate-source voltage of the transistor 110 is Vgs=Vth+Vdata sothat the transistor 110 is turned on. Therefore, a current in accordancewith luminance data flows to the transistor 110 and the light-emittingelement 116, so that the light-emitting element 116 emits light.

Note that a current I flowing to the light-emitting element 116 isrepresented by Formula (3) in the case of operating the transistor 110in a saturation region.

$\begin{matrix}\lbrack {{Formula}\mspace{14mu} 3} \rbrack & \; \\\begin{matrix}{I = {\frac{1}{2}( \frac{W}{L} )\mu\;{{Cox}( {{Vgs} - {Vth}} )}^{2}}} \\{= {\frac{1}{2}( \frac{W}{L} )\mu\;{{Cox}( {{Vth} + {Vdata} - {Vth}} )}^{2}}} \\{= {\frac{1}{2}( \frac{W}{L} )\mu\;{{Cox}({Vdata})}^{2}}}\end{matrix} & (3)\end{matrix}$

In addition, the current I flowing to the light-emitting element 116 isrepresented by Formula (4) in the case of operating the transistor 110in a linear region.

$\begin{matrix}\lbrack {{Formula}\mspace{14mu} 4} \rbrack & \; \\\begin{matrix}{I = {( \frac{W}{L} )\mu\;{{Cox}\lbrack {{( {{Vgs} - {Vth}} ){Vds}} - {\frac{1}{2}{Vds}^{2}}} \rbrack}}} \\{= {( \frac{W}{L} )\mu\;{{Cox}\lbrack {{( {{Vth} + {Vdata} - {Vth}} ){Vds}} - {\frac{1}{2}{Vds}^{2}}} \rbrack}}} \\{= {( \frac{W}{L} )\mu\;{{Cox}\lbrack {{({Vdata}){Vds}} - {\frac{1}{2}{Vds}^{2}}} \rbrack}}}\end{matrix} & (4)\end{matrix}$

Here, W denotes channel width of the transistor 110; L denotes channellength of the transistor 110; μ denotes mobility of the transistor 110;and Cox denotes storage capacitance of the transistor 110.

According to Formula (3) and Formula (4), a current flowing to thelight-emitting element 116 does not depend on the threshold voltage(Vth) of the transistor 110 in each of the case where the transistor 110is operated in the saturation region and the case where the transistor110 is operated in the linear region. Therefore, variations of a currentvalue caused by variations in the threshold voltage of the transistor110 can be suppressed, so that the current in accordance with luminancedata can be supplied to the light-emitting element 116.

Accordingly, variations in luminance caused by variations in thethreshold voltage of the transistor 110 can be suppressed. In addition,since the potential of the opposite electrode 124 is fixed at a constantpotential during the operation, power consumption can be reduced.

Further, in the case of operating the transistor 110 in the saturationregion, variations in luminance caused by deterioration of thelight-emitting element 116 can also be reduced. When the light-emittingelement 116 deteriorates, V_(EL) of the light-emitting element 116 isincreased and the potential of the first electrode, that is, the sourceelectrode of the transistor 110 rises. At this time, the sourceelectrode of the transistor 110 is connected to the second electrode ofthe capacitor 115; the gate electrode of the transistor 110 is connectedto the first electrode of the capacitor 115 and is in a floating state.Therefore, in accordance with rise in the source potential, a gatepotential of the transistor 110 rises by the same amount. Accordingly,since Vgs of the transistor 110 does not change, a current flowing tothe transistor 110 and the light-emitting element 116 is not affectedeven if the light-emitting element 116 deteriorates. Note that it can beseen in Formula (3) that the current I flowing to the light-emittingelement 116 does not depend on the source potential or a drainpotential.

Therefore, in the case of operating the transistor 110 in the saturationregion, variations in the current flowing to the transistor 110 causedby variations in the threshold voltage of the transistor 110 anddeterioration of the light-emitting element 116 can be suppressed.

Note that in the case of operating the transistor 110 in the saturationregion, as the channel length L is shorter, a larger amount of currenteasily flows by avalanche breakdown when a drain voltage is extremelyincreased.

In addition, a pinch-off point moves to a source side when the drainvoltage is increased to be higher than a pinch-off voltage, andeffective channel length which substantially functions as a channel isdecreased. Therefore, a current value is increased. Such a phenomenon isdescribed as channel length modulation. Note that the pinch-off pointmeans a boundary portion at which the channel disappears and thicknessof the channel below the gate is 0. The pinch-off voltage means avoltage when the pinch-off point is at a drain edge. This phenomenon iseasily generated as the channel length L is shorter. For example, amodel diagram of voltage-current characteristics in accordance with thechannel length modulation is shown in FIG. 4. Note that as for thechannel length of the transistors, (a)>(b)>(c) is satisfied in FIG. 4.

Accordingly, in the case of operating the transistor 110 in thesaturation region, the current I with respect to the drain-sourcevoltage Vds is preferably as constant as possible. Therefore, thechannel length L of the transistor 110 is preferably longer. Forexample, the channel length L of the transistor 110 is preferably largerthan the channel width W thereof. In addition, the channel length L ispreferably equal to or greater than 10 μm and equal to or less than 50μm. More preferably, the channel length L is equal to or greater than 15μm and equal to or less than 40 μm. Note that the channel length L andthe channel width W are not limited to them.

In addition, since a reverse bias voltage is applied to thelight-emitting element 116 in the initialization period, ashort-circuited portion in the light-emitting element 116 can beinsulated or deterioration of the light-emitting element 116 can besuppressed. Therefore, a life of the light-emitting element 116 can beextended.

Note that since variations of the current value caused by variations inthe threshold voltage of the transistor can be suppressed, a supplydestination of a current controlled by the transistor is notparticularly limited to a certain destination. Therefore, an EL element(an organic EL element, an inorganic EL element, or an EL elementincluding both an organic material and an inorganic material), anelectron-emissive element, a liquid crystal element, electronic ink, andthe like can be applied to the light-emitting element 116 shown in FIG.1.

Note that it is only necessary for the transistor 110 to have a functionfor controlling a current value supplied to the light-emitting element116, and various types of transistors can be applied to the transistor110 without particularly limiting to a certain type. Accordingly, a thinfilm transistor (TFT) using a crystalline semiconductor film, a thinfilm transistor using a non-crystalline semiconductor film typified byamorphous silicon or polycrystalline silicon, a transistor formed byusing a semiconductor substrate or an SOI substrate, a MOS transistor, ajunction transistor, a bipolar transistor, a transistor using a compoundsemiconductor such as ZnO or a-InGaZnO, a transistor using an organicsemiconductor or a carbon nanotube, or other transistors can be applied.

The first switch 111 is a switch which selects timing for inputting asignal in accordance with a gray scale of the pixel from the signal line117 into the pixel and controls a signal supplied to the gate electrodeof the transistor 110. The second switch 112 is a switch which selectstiming for supplying a predetermined potential to the gate electrode ofthe transistor 110 and controls whether to supply the predeterminedpotential to the gate electrode of the transistor 110. The third switch113 is a switch which selects timing for supplying a predeterminedpotential for initializing a potential written into the capacitor 115and lowers the potential of the first electrode of the transistor 110.The fourth switch 114 is a switch which suppresses the potentialfluctuation of the second electrode of the capacitor 115 at the time ofdata writing. Therefore, the first switch 111, the second switch 112,the third switch 113, and the fourth switch 114 are not particularlylimited as long as they have the aforementioned functions. For example,each of the first switch 111, the second switch 112, the third switch113, and the fourth switch 114 may be a transistor, a diode, or a logiccircuit combining them. Note that the first to third switches are notparticularly needed if the signal or the potential can be supplied tothe pixel at the aforementioned timing.

Next, FIG. 5 shows the case where N-channel transistors are applied tothe first switch 111, the second switch 112, the third switch 113, andthe fourth switch 114. Note that common reference numerals are used forportions which are common to the portions in the configuration in FIG. 1and the description is omitted.

A first switching transistor 511 corresponds to the first switch 111 inFIG. 1; a second switching transistor 512 corresponds to the secondswitch 112 in FIG. 1; a third switching transistor 513 corresponds tothe third switch 113 in FIG. 1; and a fourth switching transistor 514corresponds to the fourth switch 114 in FIG. 1. Channel length of thetransistor 110 is preferably longer than that of any of the firstswitching transistor 511, the second switching transistor 512, the thirdswitching transistor 513, and the fourth switching transistor 514.

A gate electrode of the first switching transistor 511 is connected tothe first scan line 118; a first electrode of the first switchingtransistor 511 is connected to the signal line 117; and a secondelectrode of the first switching transistor 511 is connected to the node131.

A gate electrode of the second switching transistor 512 is connected tothe second scan line 119; a first electrode of the second switchingtransistor 512 is connected to the power supply line 122; and a secondelectrode of the second switching transistor 512 is connected to thenode 130.

A gate electrode of the third switching transistor 513 is connected tothe third scan line 120; a first electrode of the third switchingtransistor 513 is connected to the node 132; and a second electrode ofthe third switching transistor 513 is connected to the potential supplyline 123.

A gate electrode of the fourth switching transistor 514 is connected tothe fourth scan line 121; a first electrode of the fourth switchingtransistor 514 is connected to the gate electrode of the transistor 110;and a second electrode of the fourth switching transistor 514 isconnected to the node 130.

Each of the switching transistors 511 to 514 is turned on when a signalinputted to each of the scan lines 118 to 121 is at an H level andturned off when the signal inputted to each of the scan lines 118 to 121is at an L level.

FIG. 38 shows one mode of a top plan view of the pixel shown in FIG. 5.A conductive layer 3810 includes the first scan line 118 and the gateelectrode of the first switching transistor 511. A conductive layer 3811includes the signal line 117 and the first electrode of the firstswitching transistor 511. A conductive layer 3812 includes a portionwhich functions as the second electrode of the first switchingtransistor 511, a portion which functions as the first electrode of thecapacitor 115, the second electrode of the second switching transistor512, and a portion which functions as the second electrode of the fourthswitching transistor 514. A conductive layer 3813 includes a portionwhich functions as the gate electrode of the second switching transistor512, and is connected to the second scan line 119 through a wiring 3821.A conductive layer 3814 includes a portion which functions as the firstelectrode of the second switching transistor 512 and a portion whichfunctions as the second electrode of the transistor 110, and isconnected to the power supply line 122 through a wiring 3822. Aconductive layer 3815 includes a portion which functions as the firstelectrode of the fourth switching transistor 514, and is connected to aconductive layer 3816 including a portion which functions as the gateelectrode of the transistor 110 through a wiring 3823. A conductivelayer 3817 includes a portion which functions as the gate electrode ofthe fourth switching transistor 514, and is connected to the fourth scanline 121 through a wiring 3824. A conductive layer 3818 includes aportion which functions as the first electrode of the transistor 110,and is connected to a pixel electrode 3844 of a light-emitting element.A conductive layer 3819 includes a portion which functions as the thirdscan line 120 and the gate electrode of the third switching transistor513. A conductive layer 3820 includes a portion which functions as thefirst electrode of the third switching transistor 513, and is connectedto the pixel electrode 3844. A conductive layer 3825 including a portionwhich functions as the second electrode of the third switchingtransistor 513 is connected to the potential supply line 123 through awiring 3826.

Note that the portions which function as the gate electrode, the firstelectrode, and the second electrode of the first switching transistor511 are portions which are formed by overlapping the conductive layersincluding the electrodes with a semiconductor layer 3831. The portionswhich function as the gate electrode, the first electrode, and thesecond electrode of the second switching transistor 512 are portionswhich are formed by overlapping the conductive layers including theelectrodes with a semiconductor layer 3832. The portions which functionas the gate electrode, the first electrode, and the second electrode ofthe third switching transistor 513 are portions which are formed byoverlapping the conductive layers including the electrodes with asemiconductor layer 3833. The portions which function as the gateelectrode, the first electrode, and the second electrode of the fourthswitching transistor 514 are portions which are formed by overlappingthe conductive layers including the electrodes with a semiconductorlayer 3834. Similarly, the portions which function as the gateelectrode, the first electrode, and the second electrode of thetransistor 110 are portions which are formed by overlapping theconductive layers including the electrodes with a semiconductor layer3830. Note that the capacitor 115 is formed in a portion in which theconductive layer 3813 overlaps with the pixel electrode 3844.

Also in the pixel configuration in FIG. 5, variations of the currentvalue caused by variations in the threshold voltage of the transistor110 can be suppressed by an operating method which is similar to that inFIG. 1. Therefore, the current in accordance with luminance data can besupplied to the light-emitting element 116, so that variations inluminance can be suppressed. In the case of operating the transistor 110in the saturation region, variations in luminance caused bydeterioration of the light-emitting element 116 can also be suppressed.In addition, a structure in which one of the source electrode and thedrain electrode surrounds the other of the source electrode and thedrain electrode is employed in each transistor, channel width can bewidened. Accordingly, it is more effective when a non-crystallinesemiconductor layer with lower mobility than that of a crystallinesemiconductor layer is used for a semiconductor layer of each transistorincluded in the pixel.

Further, since the pixel can be formed by using only N-channeltransistors, a manufacturing process can be simplified. In addition, anon-crystalline semiconductor such as an amorphous semiconductor or asemi-amorphous semiconductor (also described as a microcrystallinesemiconductor) can be used for the semiconductor layer of eachtransistor included in the pixel. For example, amorphous silicon(a-Si:H) can be given as an example of the amorphous semiconductor. Byusing such a non-crystalline semiconductor, the manufacturing processcan be further simplified. Accordingly, manufacturing cost can bereduced and a yield can be improved.

Note that since the first switching transistor 511, the second switchingtransistor 512, the third switching transistor 513, and the fourthswitching transistor 514 is operated just as a switch, the polarity (aconductivity type) of the transistors is not particularly limited to acertain type. However, a transistor of polarity with smaller off-currentis preferably used. A transistor provided with an LDD region, atransistor with a multi-gate structure, or the like is given as anexample of a transistor with smaller off-current. In addition, a CMOSswitch may be employed by using both N-channel and P-channeltransistors.

In addition, various connections of the switches can be employed as longas an operation which is similar to that in FIG. 1 is performed, so thatthe invention is not limited to FIG. 1. As it can be seen from FIGS. 3Ato 3D showing the operations of the pixel configuration in FIG. 1, inthe invention, it is necessary to have electrical continuity in theinitialization period, the threshold voltage writing period, datawriting period, and the light-emitting period as shown by a solid linein each of FIGS. 44A to 44D. Therefore, any configuration may beemployed as long as a switch or the like is provided so as to satisfythis and can be operated. For example, the fourth switch 114 shown inFIG. 1 may be connected between the node 130 and the node 131, and FIG.6 shows such a configuration. Note that the fourth switch 114 in FIG. 1corresponds to a fourth switch 614, and common reference numerals areused for portions which are common to the portions in the configurationin FIG. 1 and the description is omitted.

Also in the pixel configuration in FIG. 6, variations of the currentvalue caused by variations in the threshold voltage of the transistor110 can be suppressed by an operating method which is similar to that inFIG. 1. Therefore, the current in accordance with luminance data can besupplied to the light-emitting element 116, so that variations inluminance can be suppressed. In the case of operating the transistor 110in the saturation region, variations in luminance caused bydeterioration of the light-emitting element 116 can also be suppressed.

In addition, the fourth switch 114 shown in FIG. 1 may be provided on apath from the node 132 to a connection point of the second electrode ofthe transistor 110 and the power supply line 122.

FIG. 7 shows one example of such configuration. Note that a connectionpoint of the second electrode of the transistor 110 and the power supplyline 122 is denoted by a node 134. In the configuration in FIG. 7, thefourth switch 114 in FIG. 1 corresponds to a fourth switch 714 and thefourth switch 714 is connected between the second electrode of thetransistor 110 and the node 134. Note that common reference numerals areused for portions which are common to the portions in the configurationin FIG. 1 and the description is omitted.

Also in the case where the transistor 110 is turned on by the fourthswitch 714 at the time of data writing, the current flowing to thetransistor 110 can be interrupted by turning off the fourth switch 714.Therefore, fluctuation of the potential of the second electrode of thecapacitor 115 in the data writing period can be suppressed.

Accordingly, also in the pixel configuration in FIG. 7, variations ofthe current value caused by variations in the threshold voltage of thetransistor 110 can be suppressed by an operating method which is similarto that in FIG. 1. Therefore, the current in accordance with luminancedata can be supplied to the light-emitting element 116, so thatvariations in luminance can be suppressed. In addition, in the case ofoperating the transistor 110 in the saturation region, variations inluminance caused by deterioration of the light-emitting element 116 canalso be suppressed. Further, in the case of turning off the fourthswitch 114 in the initialization period, power consumption can bereduced. Note that when a connection point of the node 134 and thesecond switch 112 is denoted by a node 135, the fourth switch 714 cannotbe turned off in the initialization period in the case where the fourthswitch 714 is connected between the node 134 and the node 135 as shownin FIG. 45.

As described above, the first switch 111 is not particularly limited aslong as it is a switch which selects timing for inputting the signal inaccordance with the gray scale of the pixel from the signal line 117into the pixel and controls the signal supplied to the gate electrode ofthe transistor 110. The second switch 112 is not particularly limited aslong as it is a switch which selects timing for supplying thepredetermined potential to the gate electrode of the transistor 110 andcontrols whether to supply the predetermined potential to the gateelectrode of the transistor 110. The third switch 113 is notparticularly limited as long as it is a switch which selects timing forsupplying the predetermined potential for initializing the potentialwritten into the capacitor 115 and lowers the potential of the firstelectrode of the transistor 110. In addition, the first to thirdswitches are not particularly needed if the signal or the potential canbe supplied to the pixel at the aforementioned timing. For example, inthe case where the signal in accordance with the gray scale of the pixelcan be inputted to the pixel, the first switch 111 is not required to beprovided as shown in FIG. 16. The pixel shown in FIG. 16 includes thetransistor 110, the second switch 112, the third switch 113, the fourthswitch 114, and a pixel electrode 1640. The first electrode (one of thesource electrode and the drain electrode) of the transistor 110 isconnected to the pixel electrode 1640; a second electrode (the other ofthe source electrode and the drain electrode) of the transistor 110 isconnected to the power supply line 122 through the fourth switch 714;and a gate electrode of the transistor 110 is connected to the powersupply line 122 through the second switch 112. Further, the firstelectrode of the transistor 110 is also connected to the potentialsupply line 123. Note that since a gate capacitance 1615 of thetransistor 110 is used as the storage capacitor, the capacitor 115 inFIG. 1 is not particularly required to be provided. Also in such pixel,each switch is operated in accordance with the timing chart shown inFIG. 2 and a desired potential is supplied to each switch, so thatvariations of the current value caused by variations in the thresholdvoltage of the transistor 110 can be suppressed. That is, a desiredcurrent can be supplied to the pixel electrode 1640.

In addition, FIG. 8 shows another configuration. In the configuration inFIG. 8, the fourth switch 114 in FIG. 1 corresponds to a fourth switch814 and the fourth switch 814 is connected between the first electrodeof the transistor 110 and a node 132. Note that common referencenumerals are used for portions which are common to the portions in theconfiguration in FIG. 1 and the description is omitted.

Also in the case where the transistor 110 is turned on by the fourthswitch 814 at the time of data writing, the current flowing to the node132 can be interrupted by turning off the fourth switch 814. Therefore,fluctuation of the potential of the second electrode of the capacitor115 in the data writing period can be suppressed.

Accordingly, also in the pixel configuration in FIG. 8, variations ofthe current value caused by variations in the threshold voltage of thetransistor 110 can be suppressed by the operating method which issimilar to that in FIG. 1. Therefore, the current in accordance withluminance data can be supplied to the light-emitting element 116, sothat variations in luminance can be suppressed. In addition, in the caseof operating the transistor 110 in the saturation region, variations inluminance caused by deterioration of the light-emitting element 116 canalso be suppressed. Further, in the case of turning off the fourthswitch 114 in the initialization period, power consumption can bereduced.

Note that each of the fourth switch 614, the fourth switch 714, and thefourth switch 814 may also be a transistor, a diode, or a logic circuitcombining them, similarly to the first switch to third switches.

In addition, in the case where the fourth switch is provided on the pathfrom the node 132 to the connection point of the second electrode of thetransistor 110 and the power supply line 122 as shown in FIGS. 7 and 8,a non light-emitting state can also be forcibly made by turning off thefourth switch in the light-emitting period. By providing the nonlight-emitting period in a part of the light-emitting period by such anoperation, the light-emitting time can be freely set. Further, byinserting black display, an after image is hardly viewed and movingimage characteristics can be improved.

Next, a display device including the aforementioned pixel of theinvention is described with reference to FIG. 9.

The display device includes a signal line driver circuit 911, a scanline driver circuit 912, and a pixel portion 913. The pixel portion 913includes a plurality of signal lines S1 to Sm which are arranged whilebeing extended from the signal line driver circuit 911 in a columndirection; a plurality of first scan lines G1_1 to Gn_1, second scanlines G1_2 to Gn_2, third scan lines G1_3 to Gn_3, fourth scan linesG1_4 to Gn_4, power supply lines P1_1 to Pn_1, and potential supplylines P1_2 to Pn_2 which are arranged while being extended from the scanline driver circuit 912 in a row direction; and a plurality of pixels914 which are arranged in matrix corresponding to the signal lines S1 toSm. Each pixel 914 is connected to a signal line Sj (one of the signallines S1 to Sm), a first scan line Gi_1 (one of the scan lines G1_1 toGn_1), a second scan line Gi_2, a third scan line Gi_3, a fourth scanline Gi_4, a power supply line Pi_1, and a potential supply line Pi_2.

Note that the signal line Sj, the first scan line Gi_1, the second scanline Gi_2, the third scan line Gi_3, the fourth scan line Gi_4, thepower supply line Pi_1, and the potential supply line Pi_2 correspond tothe signal line 117, the first scan line 118, the second scan line 119,the third scan line 120, the fourth scan line 121, the power supply line122, and the potential supply line 123 in FIG. 1, respectively.

In response to a signal output from the scan line driver circuit 912,the operations shown in FIG. 2 are performed in each of pixels of onerow as well as the row of the pixels to be operated is selected. Notethat in the data writing period in FIG. 2, a video signal output fromthe signal line driver circuit 911 is written into each of the pixels ofthe selected row. At this time, a potential in accordance with luminancedata of each pixel is inputted to each of the signal lines S1 to Sm.

As shown in FIG. 10, for example, when a data writing period of an i-throw is terminated, writing of a signal into pixels in an i+1 row isperformed. Note that in order to show the data writing period of eachrow, FIG. 10 shows only the operation of the first switch 111 in FIG. 2which can precisely show the period. Then, a pixel which terminates thedata writing period in the i-th row proceeds to a light-emitting periodand emits light in accordance with the signal written into the pixel.

Therefore, unless the data writing periods of each rows overlaps, aninitialization start period can be freely set in each row. In addition,since each pixel can emit light except in its address period, a ratio ofa light-emitting period in one frame period (i.e., a duty ratio) can beextremely raised and can also be approximately 100%. Accordingly, adisplay device with few variations in luminance and a high duty ratiocan be obtained.

In addition, since the threshold voltage writing period can also be setlong, the threshold voltage of the transistor can be written in thecapacitor more accurately. Therefore, reliability as a display devicecan be improved.

Note that the configuration of the display device shown in FIG. 9 isonly one example, so that the invention is not limited to this. Forexample, the potential supply lines P1_2 to Pn_2 are not required to bearranged in parallel with the first scan lines G1_1 to Gn_1, and may bearranged in parallel with the signal lines S1 to Sm.

Meanwhile, as a driving method of the display device for expressing agray scale, there are an analog gray scale method and a digital grayscale method. The analog gray scale method includes a method whichcontrols emission intensity of a light-emitting element in an analogmanner and a method which controls light-emitting time of alight-emitting element in an analog manner. In the analog gray scalemethod, the method which controls emission intensity of a light-emittingelement in an analog manner is often used. On the other hand, in thedigital gray scale method, a gray scale is expressed by controllingon/off of a light-emitting element in a digital manner. In the case ofthe digital gray scale method, there is an advantage of high noiseresistance because data processing can be performed with a digitalsignal; however, since the digital driving method has only two states ofa light-emitting state and a non light-emitting state, the digitaldriving method can only display two gray scales by itself. Therefore,multi-gray scale display has been realized by combining with anothermethod. As a technique for multi-gray scale display, there are an areagray scale method in which light-emitting areas of pixels are weightedand selected to perform gray scale display and a time gray scale methodin which light-emitting time is weighted and selected to perform grayscale display.

In the case of combining the digital gray scale method and the time grayscale method, one frame period is divided into a plurality of subframeperiods (SFn) as shown in FIG. 43. Each subframe period includes anaddress period having an initialization period, a threshold voltagewriting period, and a data writing period, and a light-emitting period(Ts). Note that the number of the subframe periods which are provided inone frame period corresponds to the number of display bits n. Inaddition, in one frame period, a ratio of length of light-emittingperiods in respective subframe periods is set to satisfy 2^((n−1)):2^((n−2)): . . . :2:1, light-emission or non light-emission of alight-emitting element in each light-emitting period is selected, andthus, gray scales are expressed by utilizing difference in totallight-emitting time in one frame period in which the light-emittingelement emits light. In one frame period, luminance is high when thetotal light-emitting time is long, and luminance is low when the totallight-emitting time is short. Note that FIG. 43 shows an example of a4-bit gray scale, in which one frame period is divided into foursubframe periods and 2⁴=16 gray scales can be expressed by a combinationof light-emitting periods. Note that gray scales can be expressed evenwhen a ratio of length of the light-emitting periods is not apower-of-two ratio. Further, one subframe period may further be divided.

Note that in the case of realizing multi-gray scale display by using thetime gray scale method as described above, length of the light-emittingperiod of a low-order bit is short. Therefore, when a data writingoperation of the next subframe period is started immediately aftertermination of the light-emitting period, the data writing operationoverlaps with the data writing operation of a previous subframe period,so that normal operation cannot be performed. Thus, the fourth switch isprovided between the node 132 to the connection point of the secondelectrode of the transistor 110 and the power supply line 122 as shownin FIGS. 7 and 8 and a non light-emitting state is forcibly made byturning off the fourth switch in a part of the light-emitting period, sothat light emission having shorter length than data writing periodswhich are required for all rows can be expressed. Accordingly, this iseffective not only in the analog gray scale method, but also in themethod combining the digital gray scale method and the time gray scalemethod. Note also that since it is only necessary that a current doesnot flow to the light-emitting element in order to obtain the nonlight-emitting state, the non light-emitting state can be obtained bylowering the potential of the power supply line 122 or by turning on thethird switch 113, as well as turning off the fourth switch as describedabove. In addition, the non light-emitting state can also be obtained bymaking the gate-source voltage of the transistor 110 equal to or lessthan the threshold voltage thereof, and for example, the nonlight-emitting state can be obtained by additionally providing a switchin parallel with the capacitor 115 and electrically connecting betweenthe gate and the source of the transistor 110 by using the switch.

Note that variations in the threshold voltage include not only adifference between the threshold voltage of each transistor of pixels,but also include fluctuation over time in the threshold voltage in thecase of paying attention to one transistor. In addition, the differencebetween the threshold voltage of each transistor also includes adifference in transistor characteristics at the time of manufacturingeach transistor. Note that the transistor here means a transistor havinga function of supplying a current to a load such as a light-emittingelement.

Embodiment Mode 2

In this embodiment mode, FIG. 11A shows a configuration of a pixel whichis different from Embodiment Mode 1. Note that common reference numeralsare used for portions which are similar to Embodiment Mode 1 anddetailed description of the same portions or portions having similarfunctions is omitted.

A pixel shown in FIG. 11A includes the transistor 110, the first switch111, the second switch 112, the fourth switch 114, a rectifying element1113, the capacitor 115, and the light-emitting element 116. Note thatthe pixel is connected to the signal line 117, the first scan line 118,the second scan line 119, a third scan line 1120, the fourth scan line121, and the power supply line 122. The pixel shown in FIG. 11A has aconfiguration in which the rectifying element 1113 is used as the thirdswitch 113 in FIG. 3, and the second electrode of the capacitor 115, thefirst electrode of the transistor 110, and the pixel electrode of thelight-emitting element 116 are connected to the third scan line 1120through the rectifying element 1113. That is, the rectifying element1113 is connected so that a current flows from the first electrode ofthe transistor 110 to the third scan line 1120. Needless to say, asshown in Embodiment Mode 1, a transistor or the like may be used as eachof the first switch 111, the second switch 112, and the fourth switch114. In addition, diode-connected transistors 1154 and 1155 shown inFIGS. 11E and 11F can be used as the rectifying element 1113 as well asa Schottky barrier diode 1151, a PIN diode 1152, a PN diode 1153, or thelike shown in FIGS. 11B to 11D. Note that in each the transistors 1154and 1155, the polarity of the transistor is needed to be selected asappropriate depending of a direction of a current flow.

A current does not flow to the rectifying element 1113 when an H-levelsignal is inputted to the third scan line 1120, and a current flows tothe rectifying element 1113 when an L-level signal is inputted to thethird scan line 1120. Therefore, in the case of operating the pixel inFIG. 11A similarly to the pixel shown in FIG. 1, an L-level signal isinputted to the third scan line 1120 in the initialization period and anH-level signal is inputted to the third scan line 1120 in other periods.Note that since not only a current flows to the rectifying element 1113,but also the potential of the second electrode of the capacitor 115 isrequired to be lowered to V1−Vth−α (α: an arbitrary positive number), apotential of the L-level signal is V1−Vth−α−β (α, β: an arbitrarypositive number). Note that β shows the threshold voltage in a forwarddirection of the rectifying element 1113. Further, the L-level signalmay be made lower than the potential of the opposite electrode 124 ofthe light-emitting element and a reverse bias voltage may be applied tothe light-emitting element 116 in the initialization period. On theother hand, since the H-level signal is not particularly limited as longas a current does not flow to the rectifying element 1113 as describedabove, the H-level signal may be any signal which is larger than a valueobtained by subtracting the threshold voltage of the rectifying element1113 from V1−Vth, that is, V1−Vth−β.

Considering the aforementioned description, by performing an operationwhich is similar to that in Embodiment Mode 1 also in the pixelconfiguration in FIG. 11A, variations in a current value caused byvariations in the threshold voltage of the transistor 110 can besuppressed. Therefore, a current in accordance with luminance data canbe supplied to the light-emitting element 116, so that variations inluminance can be suppressed. In addition, in the case of operating thetransistor 110 in the saturation region, variations in luminance causedby deterioration of the light-emitting element 116 can also besuppressed. Further, by using the rectifying element 1113, the number ofwirings can be reduced, so that an aperture ratio can be improved.

In addition, the pixel shown in this embodiment mode can be applied tothe display device in FIG. 9. Similarly to Embodiment Mode 1, unless thedata writing periods of each rows overlaps, an initialization startperiod can be freely set in each row. Further, since each pixel can emitlight except in its address period, a ratio of a light-emitting periodin one frame period (i.e., a duty ratio) can be extremely raised and canalso be approximately 100%. Accordingly, a display device with fewvariations in luminance and a high duty ratio can be obtained.

In addition, since a threshold voltage writing period can also be setlong, the threshold voltage of a transistor which controls a currentvalue flowing to the light-emitting element can be written in thecapacitor more accurately. Therefore, reliability as a display devicecan be improved.

This embodiment mode can also be freely combined with any pixelconfiguration shown in another embodiment mode in addition to the pixelconfiguration in FIG. 1. For example, the case where the fourth switch114 is connected between the node 130 and the node 131 or between thefirst electrode of the transistor 110 and the node 132 and the casewhere the second electrode of the transistor 110 is connected to thepower supply line 122 through the fourth switch 114 are given asexamples. The invention is not limited to this, and the rectifyingelement 1113 can also be applied to the pixels shown in other embodimentmodes.

Embodiment Mode 3

In this embodiment mode, FIGS. 12 to 15 show configurations each a pixelwhich is different from Embodiment Modes 1 and 2. Note that althoughdescription is made by paying attention to one pixel in Embodiment Modes1 and 2, the number of wirings can be reduced by sharing a wiringconnected to each pixel among pixels. In this case, if normal operationcan be performed, various wirings can be shared among the pixels. Forexample, a wiring can be shared with the next pixel and this embodimentmode shows one example of the method. Note that common referencenumerals are used for portions which are similar to Embodiment Mode 1and detailed description of the same portions or portions having similarfunctions is omitted.

A pixel 1200 shown in FIG. 12 includes the transistor 110, the firstswitch 111, the second switch 112, the third switch 113, the fourthswitch 114, the capacitor 115, and the light-emitting element 116. Notethat the pixel 1200 is connected to the signal line 117, a first scanline 1218, the second scan line 119, the third scan line 120, the fourthscan line 121, the power supply line 122, and a first scan line 1218 inthe following row.

Although the second electrode of the capacitor 115 is connected to thepotential supply line 123 through the third switch 113 in the pixelshown in FIG. 1 in Embodiment Mode 1, the second electrode of thecapacitor 115 can be connected to the first scan line 1218 of thefollowing row in FIG. 12. This is because it is only necessary that apredetermined potential is supplied to the second electrode of thecapacitor 115 in the initialization period, without limiting to thepotential supply line 123. Therefore, as long as the predeterminedpotential can be supplied to the second electrode of the capacitor 115in the initialization period, a wiring which supplies a potential is notalways required to have a constant potential. Thus, the first scan line1218 of the following row can be used instead of the potential supplyline 123. By sharing the wiring with the following row in this manner,the number of wirings can be reduced, so that an aperture ratio can beimproved.

Note that by performing operations similar to those in Embodiment Mode 1also in the pixel configuration in FIG. 12, variations in a currentvalue caused by variations in the threshold voltage of the transistor110 can be suppressed. Therefore, a current in accordance with luminancedata can be supplied to the light-emitting element 116, so thatvariations in luminance can be suppressed. In addition, since thetransistor 110 is operated with a potential of an opposite electrodefixed at a constant potential, power consumption can be reduced. Notethat although an operation region of the transistor 110 is notparticularly limited, an advantageous effect of the invention becomesmore apparent when the transistor 110 is operated in the saturationregion. Further, in the case of operating the transistor 110 in thesaturation region, variations in a current flowing to the transistor 110caused by deterioration of the light-emitting element 116 can besuppressed.

Note that a signal turning off the first switch 111 in the first scanline 1218 is a potential of V1−Vth−α (α: an arbitrary positive number).Therefore, it is necessary to use the first switch 111 which is turnedoff by the potential of V1−Vth−α (α: an arbitrary positive number). Inaddition, it is necessary to operate such that the initialization periodof a row of the pixel 1200 does not overlap with the data writing periodof a row which shares the wiring with the row of the pixel 1200.

Note that in the case of using an N-channel transistor as the thirdswitch 113, a potential which turns off the third switch 113 in thethird scan line 120 may be lower than the potential of V1−Vth−α which isthe signal turning off the first switch 111 in the first scan line 1218.In this case, a gate-source voltage of the transistor in an off statecan be made a negative value, so that current leakage when the thirdswitch 113 is off can be reduced.

Although the potential of V1−Vth−α is used as the signal turning off thefirst switch 111 in the aforementioned description, it may also be usedas a signal turning on the first switch 111. Note that limitation of theoperations is different from that in the aforementioned description.

In addition, as shown in a pixel 1300 in FIG. 13, the potential supplyline 123 in FIG. 1 may be shared with a second scan line 1319 of thefollowing row. Also in the pixel 1300, operations which are similar tothose in Embodiment Mode 1 can be performed. Note that it is preferablethat a signal turning off the second switch 112 in the second scan line1319 be a potential of V1−Vth−α (α: an arbitrary positive number). Inthis case, it is necessary to use the second switch 112 which is turnedoff by the potential of V1−Vth−α (α: an arbitrary positive number). Inaddition, it is necessary to operate such that the initialization periodof a row of the pixel 1300 does not overlap with the data writing periodof a row which shares the wiring with the row of the pixel 1300.

Note that in the case of using an N-channel transistor as the thirdswitch 113, a potential which turns off the third switch 113 in thethird scan line 120 may be lower than the potential of V1−Vth−α which isthe signal turning off the second switch 112 in the second scan line1319. In this case, current leakage when the third switch 113 is off canbe reduced.

Although the potential of V1−Vth−α is used as the signal turning off thesecond switch 112 in the aforementioned description, it may also be usedas a signal turning on the second switch 112. Note that limitation ofthe operations is different from that in the aforementioned description.

In addition, as shown in a pixel 1400 in FIG. 14, the potential supplyline 123 in FIG. 1 may be shared with a third scan line 1420 of aprevious row. Also in the pixel 1400, operations which are similar tothose in Embodiment Mode 1 can be performed. Note that a signal turningoff the third switch 113 in the third scan line 1420 is a potential ofV1−Vth−α (α: an arbitrary positive number). Therefore, it is necessaryto use the third switch 113 which is turned off by the potential ofV1−Vth−α (α: an arbitrary positive number). In this case, although it isnecessary to operate such that the initialization period of a row of thepixel 1400 does not overlap with the initialization period of a rowwhich shares a wiring with the row of the pixel 1400, it does notparticularly matter when the initialization period is set to be shorterthan the data writing period.

Although the potential of V1−Vth−α is used as the signal turning off thethird switch 113 in the aforementioned description, it may also be usedas a signal turning on the third switch 113. Note that limitation of theoperations is different from that in the aforementioned description.

In addition, as shown in a pixel 1500 in FIG. 15, the potential supplyline 123 in FIG. 1 may be shared with a fourth scan line 1521 of thefollowing row. Also in the pixel 1500, operations which are similar tothose in Embodiment Mode 1 can be performed. Note that in the fourthscan line 1521, it is preferable that the fourth switch 114 which isturned on when a potential of V1−Vth−α (α: an arbitrary positive number)is input thereinto be used. In this case, it is necessary to operatesuch that the initialization period of a row of the pixel 1500 does notoverlap with the data writing period of a row which shares the wiringwith the row of the pixel 1500. Further, in the case of turning off thefourth switch 114 in the initialization period, it is necessary tooperate such that the initialization period of the row of the pixel 1500does not overlap with the initialization period of the row which sharesthe wiring with the row of the pixel 1500.

Although the potential of V1−Vth−α is used as the signal turning on thefourth switch 114 in the aforementioned description, it may also be usedas a signal turning off the fourth switch 114. Note that limitation ofthe operations is different from that in the aforementioned description.

In addition to the aforementioned description, the potential supply line123 in FIG. 1 may be shared with the power supply line 122 of thefollowing row. In that case, three kinds of potentials V1, V2, andV1−Vth−α (α: an arbitrary positive number) are supplied to the powersupply line 122, and a pixel configuration in which operations which aresimilar to those in Embodiment mode 1 can be performed may be employed.

Although the case is described in which the potential supply line 123 inFIG. 1 is shared with the scan line of the following row or the previousrow in this embodiment mode, another wiring may be used as long as itcan supply a potential of V1−Vth−α (α: an arbitrary positive number) inthe initialization period.

Further, pixel shown in this embodiment mode can be applied to thedisplay device in FIG. 9. An initialization start period can be freelyset in each row within a limitation of the operations in each pixelshown in FIGS. 12 to 15 and a range in which the data writing period ineach row does not overlap. In addition, since each pixel can emit lightexcept in its address period, a ratio of a light-emitting period in oneframe period (i.e., a duty ratio) can be extremely raised and can alsobe approximately 100%. Accordingly, a display device with few variationsin luminance and a high duty ratio can be obtained.

In addition, since a threshold voltage writing period can also be setlong, the threshold voltage of a transistor which controls a currentvalue flowing to the light-emitting element can be written in thecapacitor more accurately. Therefore, reliability as a display devicecan be improved.

The fourth switch 114 is not necessarily connected between the node 130and the gate electrode of the transistor 110, and may be connectedbetween the node 130 and the node 131 or the first electrode of thetransistor 110 and the node 132. In addition, the second electrode ofthe transistor 110 may be connected to the power supply line 122 throughthe fourth switch 114.

This embodiment mode can be freely combined with any pixel configurationshown in another embodiment mode, without limiting to the aforementioneddescription.

Embodiment Mode 4

In this embodiment mode, FIG. 29 shows a configuration of a pixel whichis different from Embodiment Mode 1. Note that common reference numeralsare used for portions which are similar to Embodiment Mode 1 anddetailed description of the same portions or portions having similarfunctions is omitted.

A pixel shown in FIG. 29 includes a transistor 2910, the first switch111, the second switch 112, the third switch 113, the fourth switch 114,the capacitor 115, and the light-emitting element 116. Note that thepixel is connected to the signal line 117, the first scan line 118, thesecond scan line 119, the third scan line 120, the fourth scan line 121,the power supply line 122, and the potential supply line 123.

The transistor 2910 in this embodiment mode is a multi-gate transistorwhere two transistors are connected in series, and is provided in thesame position as that of the transistor 110 in Embodiment Mode 1. Notethat the number of transistors which are connected in series is notparticularly limited.

By performing operations similar to those of the pixel in FIG. 1 in thepixel in FIG. 29, variations of a current value caused by variations inthe threshold voltage of the transistor 2910 can be suppressed.Therefore, a current in accordance with luminance data can be suppliedto the light-emitting element 116, so that variations in luminance canbe suppressed. In addition, since the transistor 2910 is operated with apotential of an opposite electrode fixed at a constant potential, powerconsumption can be reduced. Note that although an operation region ofthe transistor 2910 is not particularly limited, an advantageous effectof the invention becomes more apparent when the transistor 2910 isoperated in the saturation region.

Further, in the case of operating the transistor 2910 in the saturationregion, variations of the currents flowing to the transistor 2910 causedby deterioration of the light-emitting element 116 can be suppressed.

When channel widths of the two transistors connected in series are equalto each other, channel length L of the transistor 2910 in thisembodiment mode is equal to the sum of the channel widths of the twotransistors. Therefore, a current value which is closer to a constantvalue can be easily obtained in the saturation region regardless of adrain-source voltage Vds. In particular, the transistor 2910 iseffective when it is difficult to manufacture a transistor having longchannel length L. Note that a connection portion of the two transistorsfunctions as a resistor.

Note that it is only necessary for the transistor 2910 to have afunction for controlling a current value supplied to the light-emittingelement 116, and a type of the transistor 2910 is not particularlylimited. Accordingly, a thin film transistor (TFT) using a crystallinesemiconductor film, a thin film transistor using a non-crystallinesemiconductor film typified by amorphous silicon or polycrystallinesilicon, a transistor formed by using a semiconductor substrate or anSOI substrate, a MOS transistor, a junction transistor, a bipolartransistor, a transistor using a compound semiconductor such as ZnO ora-InGaZnO, a transistor using an organic semiconductor or a carbonnanotube, or other transistors can be applied.

In addition, in the pixel shown in FIG. 29, a transistor or the like canbe used as each of the first switch 111, the second switch 112, thethird switch 113, and the fourth switch 114, similarly to the pixelshown in FIG. 1,

Note that the switch 114 is not necessarily connected between the node130 and a gate electrode of the transistor 2910, and may be connectedbetween the node 130 and the node 131 or a first electrode of thetransistor 2910 and the node 132. In addition, a second electrode of thetransistor 2910 may be connected to the power supply line 122 throughthe fourth switch 114.

In addition, the pixel shown in this embodiment mode can be applied tothe display device in FIG. 9. Similarly to Embodiment Mode 1, unless thedata writing period in each row overlaps, an initialization start periodcan be freely set in each row. Further, since each pixel can emit lightexcept in its address period, a ratio of a light-emitting period in oneframe period (i.e., a duty ratio) can be extremely raised and can alsobe approximately 100%. Accordingly, a display device with few variationsin luminance and a high duty ratio can be obtained.

In addition, since a threshold voltage writing period can also be setlong, the threshold voltage of a transistor which controls a currentvalue flowing to the light-emitting element can be written in thecapacitor more accurately. Therefore, reliability as a display devicecan be improved.

Note that the transistor 2910 is not limited to a structure wheretransistors are connected in series, and may be a structure wheretransistors are connected in parallel like a transistor 3010 shown inFIG. 30. A larger current can be supplied to the light-emitting element116 by using the transistor 3010. In addition, since transistorcharacteristics are averaged by using the two transistors connected inparallel, original variations in characteristics of the transistorsincluded in the transistor 3010 can be more reduced. Therefore, whenvariations are reduced, variations of the current value caused byvariations in the threshold voltage of the transistor can be easilysuppressed.

Further, each of the transistors connected in parallel shown in thetransistor 3010 may be connected in series like the transistor 2910shown in FIG. 29.

This embodiment mode can be freely combined with any pixel configurationshown in another embodiment mode, without limiting to the aforementioneddescription. That is, the transistor 2910 or the transistor 3010 can beapplied to any of pixel configurations shown in other embodiment modes

Embodiment Mode 5

In this embodiment mode, a pixel configuration is described in whichdeterioration of transistors over time is averaged by switchingtransistors which control a current value supplied to a light-emittingelement for each period in the pixel of the invention, with reference toFIG. 31.

A pixel shown in FIG. 31 includes a first transistor 3101, a secondtransistor 3102, a first switch 3111, a second switch 3112, a thirdswitch 3113, a fourth switch 3114, a fifth switch 3103, a sixth switch3104, a capacitor 3115, and a light-emitting element 3116. Note that thepixel is connected to a signal line 3117, a first scan line 3118, asecond scan line 3119, a third scan line 3120, a fourth scan line 3121,a power supply line 3122, and a potential supply line 3123. Further,although not shown in FIG. 31, the pixel is connected to fifth and sixthscan lines which control on/off of the fifth switch 3103 and the sixthtransistor 3104, respectively. In this embodiment mode, each of thefirst transistor 3101 and the second transistor 3102 is an N-channeltransistor, and is turned on when a gate-source voltage (Vgs) thereofexceeds the threshold voltage. In addition, a pixel electrode of thelight-emitting element 3116 corresponds to an anode and an oppositeelectrode 3124 thereof corresponds to a cathode. Note that a gate-sourcevoltage of the transistor is described as Vgs and a voltage stored inthe capacitor is described as Vcs. Further, the threshold voltage of thefirst transistor 3101 is described as Vth1 and the threshold voltage ofthe second transistor 3102 is described as Vth2. The power supply line3122, the potential supply line 3123, and the signal line 3117 are alsodescribed as a first wiring, a second wiring, and a third wiring,respectively.

A first electrode (one of a source electrode and a drain electrode) ofthe first transistor 3101 is connected to the pixel electrode of thelight-emitting element 3116 through the fifth switch 3103; a secondelectrode (the other of the source electrode and the drain electrode) ofthe first transistor 3101 is connected to the power supply line 3122;and a gate electrode of the first transistor 3101 is connected to thepower supply line 3122 through the fourth switch 3114 and the secondswitch 3112. Note that the fourth switch 3114 is connected between thegate electrode of the first transistor 3101 and the second switch 3112.In addition, if a connection point of the fourth switch 3114 and thesecond switch 3112 is denoted by a node 3130, the node 3130 is connectedto the signal line 3117 through the first switch 3111. Further, thefirst electrode of the first transistor 3101 is also connected to thepotential supply line 3123 through the fifth switch 3103 and the thirdswitch 3113.

A first electrode (one of a source electrode and a drain electrode) ofthe second transistor 3102 is connected to the pixel electrode of thelight-emitting element 3116 through the sixth switch 3104; a secondelectrode (the other of the source electrode and the drain electrode) ofthe second transistor 3102 is connected to the power supply line 3122;and a gate electrode of the second transistor 3102 is connected to thenode 3130 through the fourth switch 3114. In addition, the firstelectrode of the second transistor 3102 is also connected to thepotential supply line 3123 through the sixth switch 3104 and the thirdswitch 3113. Note that the gate electrode of the first transistor 3101and the gate electrode of the second transistor 3102 are connected.Further, the first electrode of the first transistor 3101 and the firstelectrode of the second transistor 3102 are connected through the fifthswitch 3103 and the sixth switch 3104, and a connection point of thefifth switch 3103 and the sixth switch 3104 is denoted by a node 3133.

In addition, the capacitor 3115 is connected between the node 3133 andthe node 3130. That is, a first electrode of the capacitor 3115 isconnected to the gate electrodes of the first transistor 3101 and thesecond transistor 3102 through the fourth switch 3114; and a secondelectrode of the capacitor 3115 is connected to the first electrode ofthe first transistor 3101 through the fifth switch 3103 and is connectedto the first electrode of the second transistor 3102 through the sixthswitch 3104. The capacitor 3115 may be formed by sandwiching aninsulating film with a wiring, a semiconductor layer, or an electrode,or can be omitted by using gate capacitance of the first transistor 3101and the second transistor 3102 in some cases. Further, a connectionpoint of the first electrode of the capacitor 3115 and a wiring to whichthe first switch 3111 and the node 3130 are connected is denoted by anode 3131, and a connection point of a wiring to which the node 3133 andthe second electrode of the capacitor 3115 are connected and the pixelelectrode of the light-emitting element 3116 is denoted by a node 3132.

Note that by inputting signals into the first scan line 3118, the secondscan line 3119, the third scan line 3120, and the fourth scan line 3121,on/off of the first switch 3111, the second switch 3112, the thirdswitch 3113, and the fourth switch 3114 is controlled, respectively. InFIG. 31, scan lines which control on/off of the fifth switch 3103 andthe sixth switch 3104 respectively are omitted.

A signal in accordance with a gray scale of the pixel which correspondsto a video signal, that is, a potential in accordance with luminancedata is inputted to the signal line 3117.

Next, operations of the pixel shown in FIG. 31 are described withreference to a timing chart in FIG. 32. Note that, in FIG. 32, one frameperiod which corresponds to a period for displaying an image for onescreen is divided into an initialization period, a threshold voltagewriting period, a data writing period, and a light-emitting period.

Note that a potential of V1 is inputted to the opposite electrode 3124of the light-emitting element 3116 and a potential of V1−Vth−α (α: anarbitrary positive number) is inputted to the potential supply line3123. Vth corresponds to a higher potential between Vth1 and Vth2. Inaddition, the potential of V1 is inputted to the power supply line 3122in the address period, and a potential of V2 is inputted to the powersupply line 3122 in the light-emitting period. Note that V2>V1 issatisfied.

Here, although a potential of the opposite electrode 3124 of thelight-emitting element 3116 is the same as a potential of the powersupply line 3122 in the address period for description of theoperations, the potential of the opposite electrode 3124 may be anypotential as long as it is higher than a potential of V1−Vth−α−V_(EL)when a potential difference which is at least necessary for thelight-emitting element 3116 to emit light is V_(EL). In addition, thepotential V2 of the power supply line 3122 in the light-emitting periodmay be any potential as long as it is higher than the sum of thepotential of the opposite electrode 3124 and the potential difference(V_(EL)) which is at least necessary for the light-emitting element 3116to emit light; here, since the potential of the opposite electrode 3124is V1 for description, V2 may be any potential higher than V1+V_(EL).

First, in the initialization period, the first switch 3111 and the sixthswitch 3104 are turned off and the second switch 3112, the third switch3113, the fourth switch 3114, and the fifth switch 3103 are turned on asshown (A) in FIG. 32. At this time, the first electrode of the firsttransistor 3101 is the source electrode, and a potential thereof isequal to a potential of the potential supply line 3123 which isV1−Vth−α. On the other hand, a potential of the gate electrode of thefirst transistor 3101 is V1. Therefore, a gate-source voltage Vgs of thefirst transistor 3101 is Vth+α so that the first transistor 3101 isturned on. Then, Vth+α is held in the capacitor 3115 provided betweenthe gate electrode and the first electrode of the first transistor 3101.Although the case where the fourth switch 3114 is turned on isdescribed, the fourth switch 3114 may be turned off. Note that in thenext threshold voltage writing period, the fourth switch 114 is requiredto be turned on.

In the threshold voltage writing period shown (B) in FIG. 32, the thirdswitch 3113 is turned off. Therefore, the potential of the firstelectrode, that is, the source electrode of the first transistor 3101rises gradually, and when the potential reaches V1−Vth1, that is, whenthe gate-source voltage Vgs of the first transistor 3101 reaches thethreshold voltage (Vth1), the first transistor 3101 is turned off.Accordingly, a voltage held in the capacitor 3115 is Vth1.

In the next data writing period shown (C) in FIG. 32, the first switch3111 is turned on after the second switch 3112 and the fourth switch3114 are turned off, and a potential in accordance with luminance data(V1+Vdata) is inputted from the signal line 3117. By turning off thefourth switch 3114 in this period, the first transistor 3101 can be heldto be turned off. Therefore, potential fluctuation of the secondelectrode of the capacitor 3115 caused by a current supplied from thepower supply line 3122 at the time of data writing can be suppressed.Accordingly, a voltage Vcs which is held in the capacitor 3115 at thistime is Vth1+Vdata. Note that in the case where the light-emittingelement 3116 is controlled not to emit light in the next light-emittingperiod, a potential of Vdata≦0 is input.

Next, in the light-emitting period shown (D) in FIG. 32, the fourthswitch 3114 is turned on after the first switch 3111 is turned off andthe potential of the power supply line 3122 is made V2. At this time, agate-source voltage of the first transistor 3101 is Vgs=Vth1+Vdata sothat the first transistor 3101 is turned on. Therefore, the current inaccordance with luminance data flows to the first transistor 3101 andthe light-emitting element 3116, so that the light-emitting element 3116emits light.

By performing such an operation, a current flowing to the light-emittingelement 3116 does not depend on the threshold voltage (Vth1) of thefirst transistor 3101 in each of the case where the first transistor3101 is operated in the saturation region and the case where the firsttransistor 3101 is operated in the linear region.

Further, in the initialization period of the next one frame period (E)shown in FIG. 32, the fifth switch 3103 is turned off and the secondswitch 3112, the third switch 3113, the fourth switch 3114, and thesixth switch 3104 are turned on. At this time, the first electrode ofthe second transistor 3102 is the source electrode, and a potentialthereof is equal to the potential of the potential supply line 3123which is V1−Vth−α. On the other hand, a potential of the gate electrodeof the second transistor 3102 is V1. Therefore, a gate-source voltageVgs of the second transistor 3102 is Vth+α so that the second transistor3102 is turned on. Then, Vth+α is held in the capacitor 3115 providedbetween the gate electrode and the first electrode of the secondtransistor 3102. Although the case where the fourth switch 3114 isturned on is described, the fourth switch 3114 may be turned off. Notethat in the next threshold voltage writing period, the fourth switch3114 is required to be turned on.

Next, in the threshold voltage writing period (F) shown in FIG. 32, thethird switch 3113 is turned off. Therefore, the potential of the firstelectrode, that is, the source electrode of the second transistor 3102rises gradually, and when the potential reaches V1−Vth2, that is, whenthe gate-source voltage Vgs of the second transistor 3102 reaches thethreshold voltage (Vth2), the second transistor 3102 is turned off.Accordingly, the voltage Vcs which is held in the capacitor 3115 isVth2.

In the following data writing period (G) shown in FIG. 32, the firstswitch 3111 is turned on after the second switch 3112 and the fourthswitch 3114 are turned off, and the potential in accordance withluminance data (V1+Vdata) is inputted from the signal line 3117. Byturning off the fourth switch 3114 in this period, the second transistor3102 can be held to be turned off. Therefore, potential fluctuation ofthe second electrode of the capacitor 3115 caused by the currentsupplied from the power supply line 3122 at the time of data writing canbe suppressed. Accordingly, the voltage Vcs which is held in thecapacitor 3115 at this time is Vth2+Vdata.

Next, in the light-emitting period (H) shown in FIG. 32, the fourthswitch 3114 is turned on after the first switch 3111 is turned off andthe potential of the power supply line 3122 is made V2. At this time, agate-source voltage of the second transistor 3102 is Vgs=Vth2+Vdata sothat the second transistor 3102 is turned on. Therefore, the current inaccordance with luminance data flows to the second transistor 3102 andthe light-emitting element 3116, so that the light-emitting element 3116emits light.

The current flowing to the light-emitting element 3116 does not dependon the threshold voltage (Vth2) of the second transistor 3102 in each ofthe case where the second transistor 3102 is operated in the saturationregion and the case where the second transistor 3102 is operated in thelinear region.

Therefore, in the case of controlling a current supplied to thelight-emitting element by using either the first transistor 3101 or thesecond transistor 3102, variations of the current value caused byvariations in the threshold voltage of the transistor can be suppressed,so that the current in accordance with luminance data can be supplied tothe light-emitting element 3116. Note that by switching the firsttransistor 3101 and the second transistor 3102, a load added to onetransistor is reduced, and thus, fluctuation of the threshold voltage ofthe transistor over time can be decreased.

Accordingly, variations in luminance caused by variations in thethreshold voltage of each of the first transistor 3101 and the secondtransistor 3102 can be suppressed. In addition, since the potential ofthe opposite electrode 3124 is fixed at a constant potential, powerconsumption can be reduced.

Therefore, in the case of operating the first transistor 3101 and thesecond transistor 3102 in the saturation region, variations in a currentflowing to each of the first transistor 3101 and the second transistor3102 caused by deterioration of the light-emitting element 3116 can besuppressed.

Note that in the case of operating the first transistor 3101 and thesecond transistor 3102 in the saturation region, channel length L ofeach transistor is preferably long.

In addition, since a reverse bias voltage is applied to thelight-emitting element 3116 in the initialization period, ashort-circuited portion in the light-emitting element 3116 can beinsulated or deterioration of the light-emitting element 3116 can besuppressed. Therefore, a life of the light-emitting element 3116 can beextended.

Note that since variations of the current value caused by variations inthe threshold voltage of the transistor can be suppressed, a supplydestination of a current controlled by the transistor is notparticularly limited to a certain destination. Therefore, an EL element(an organic EL element, an inorganic EL element, or an EL elementincluding both an organic material and an inorganic material), anelectron-emissive element, a liquid crystal element, electronic ink, andthe like can be applied to the light-emitting element 3116 shown in FIG.31.

Note that it is only necessary for each of the first transistor 3101 andthe second transistor 3102 to have a function for controlling a currentvalue supplied to the light-emitting element 3116, and a type of each ofthe first transistor 3101 and the second transistor 3102 is notparticularly limited. Accordingly, a thin film transistor (TFT) using acrystalline semiconductor film, a thin film transistor using anon-crystalline semiconductor film typified by amorphous silicon orpolycrystalline silicon, a transistor formed by using a semiconductorsubstrate or an SOI substrate, a MOS transistor, a junction transistor,a bipolar transistor, a transistor using a compound semiconductor suchas ZnO or a-InGaZnO, a transistor using an organic semiconductor or acarbon nanotube, or other transistors can be applied.

The first switch 3111 is a switch which selects timing for inputting asignal in accordance with a gray scale of the pixel from the signal line3117 into the pixel. The second switch 3112 is a switch which selectstiming for supplying a predetermined potential to the gate electrode ofthe first transistor 3101 or the second transistor 3102. The thirdswitch 3113 is a switch which selects timing for supplying apredetermined potential for initializing a potential written into thecapacitor 3115. The fourth switch 3114 is a switch which suppressespotential fluctuation of the second electrode of the capacitor 3115 atthe time of data writing. Therefore, the first switch 3111, the secondswitch 3112, the third switch 3113, and the fourth switch 3114 are notparticularly limited as long as they have the aforementioned functions.For example, each of the first switch 3111, the second switch 3112, thethird switch 3113, and the fourth switch 3114 may be a transistor, adiode, or a logic circuit combining them. In addition, the fifth switch3103 and the sixth switch 3104 are not particularly limited. Forexample, each of the fifth switch 3103, and the sixth switch 3104 may bea transistor, a diode, or a logic circuit combining them.

Further, since the pixel can be formed by using only N-channeltransistors when N-channel transistors are used for the first switch3111, the second switch 3112, the third switch 3113, the fourth switch3114, the fifth switch 3103, and the sixth switch 3104, a manufacturingprocess can be simplified. In addition, a non-crystalline semiconductorsuch as an amorphous semiconductor or a semi-amorphous semiconductor(also described as a microcrystalline semiconductor) can be used for thesemiconductor layer of each transistor included in the pixel. Forexample, amorphous silicon (a-Si:H) can be given as an example of theamorphous semiconductor. By using such a non-crystalline semiconductor,the manufacturing process can be further simplified. Accordingly,manufacturing cost can be reduced and a yield can be improved.

Note that when a transistor is used for each of the first switch 3111,the second switch 3112, the third switch 3113, the fourth switch 3114,the fifth switch 3103, and the sixth switch 3104, the polarity (aconductivity type) of each transistor is not particularly limited to acertain type. However, a transistor of polarity with smaller off-currentis preferably used.

In addition, the first transistor 3101 and the fifth switch 3103, andthe second transistor 3102 and the sixth transistor 3104 may be switchedto be arranged, as shown in FIG. 37. That is, the first electrodes ofthe first transistor 3101 and the second transistor 3102 are connectedto the gate electrodes of the first transistor 3101 and the secondtransistor 3102 through the capacitor 3115 and the fourth switch 3114.Further, the second electrode of the first transistor 3101 is connectedto the power supply line 3122 through the fifth switch 3103, and thesecond electrode of the second transistor 3102 is connected to the powersupply line 3122 through the sixth switch 3104.

Furthermore, although FIGS. 31 and 37 show the cases where the number ofsets arranged in parallel is two, using a transistor and a switch as oneset, that is, using the first transistor 3101 and the fifth switch 3103as a set, and using the second transistor 3102 and the sixth switch 3104as a set, the number of sets arranged in parallel is not particularlylimited.

Note that the fourth switch 3114 is not necessarily connected betweenthe node 3130 and the gate electrode of the first transistor 3101, andmay be connected between the node 3130 and the node 3131 or the node3133 and the node 3132.

In addition, the fourth switch 3114 is not necessarily to be provided asshown in FIG. 42. In the pixel shown in this embodiment mode, a currentsupplied from the power supply line 3122 to the node 3133 can beinterrupted by turning off both of the fifth switch 3103 and the sixthswitch 3104 in the data writing period even when the fourth switch 3114is not provided. Therefore, since the potential fluctuation of thesecond electrode of the capacitor 3115 can be suppressed, a voltage ofVth1+Vdata or a voltage of Vth2+Vdata can be held in the capacitor 3115,without particularly providing the fourth switch 3114. Accordingly, amore accurate current in accordance with luminance data can be suppliedto the light-emitting element 3116 in the light-emitting period withoutusing the fourth switch 3114. Needless to say, this can also be true forthe pixel shown in FIG. 31, that is, the case where the fifth switch3103 is connected between the first electrode of the first transistor3101 and the node 3133, and the sixth switch 3104 is connected betweenthe first electrode of the second transistor 3102 and the node 3133.

In addition, a non light-emitting state can also be forcibly made byturning off both of the fifth switch 3103 and the sixth switch 3104 inthe light-emitting period. By performing such an operation, thelight-emitting time can be freely set. Further, by inserting blackdisplay, an after image is hardly viewed and moving imagecharacteristics can be improved.

In addition, the pixel shown in this embodiment mode can be applied tothe display device in FIG. 9. Similarly to Embodiment Mode 1, unless thedata writing period in each row overlaps, an initialization start periodcan be freely set in each row. Further, since each pixel can emit lightexcept in its address period, a ratio of a light-emitting period in oneframe period (i.e., a duty ratio) can be extremely raised and can alsobe approximately 100%. Accordingly, a display device with few variationsin luminance and a high duty ratio can be obtained.

In addition, since the threshold voltage writing period can also be setlong, the threshold voltage of a transistor which controls a currentvalue flowing to the light-emitting element can be written in thecapacitor more accurately. Therefore, reliability as a display devicecan be improved.

Note that also in this embodiment mode, the potential supply line 3123can be shared with a wiring of another row, similarly to Embodiment Mode4. In addition, a multi-gate transistor where transistors are connectedin series or a transistor where transistors are arranged in parallel maybe used as each of the first transistor 3101 and the second transistor3102. This embodiment mode is not limited to them, and can be applied toany of the pixel configurations shown in Embodiment Modes 1 to 4.

Embodiment Mode 6

In this embodiment mode, the case is described in which a P-channeltransistor is employed as a transistor which controls a current valuesupplied to a light-emitting element, with reference to FIG. 39.

A pixel shown in FIG. 39 includes a transistor 3910, a first switch3911, a second switch 3912, a third switch 3913, a fourth switch 3914, acapacitor 3915, and a light-emitting element 3916. Note that the pixelis connected to a signal line 3917, a first scan line 3918, a secondscan line 3919, a third scan line 3920, a fourth scan line 3921, a powersupply line 3922, and a potential supply line 3923. In this embodimentmode, the transistor 3910 is a P-channel transistor, and is turned onwhen the absolute value of a gate-source voltage (|Vgs|) thereof exceedsthe threshold voltage (|Vth|) (when Vgs is lower than Vth). In addition,an example is described in which an EL element in which a current issupplied from a pixel electrode 4911 to an opposite electrode 3924 isused for the light-emitting element 3916 as shown in FIG. 49. In thatcase, the pixel electrode 4911 functions as an anode and the oppositeelectrode 3924 functions as a cathode. Note that the absolute value of agate-source voltage of the transistor is described as |Vgs|, and theabsolute value of the threshold voltage of the transistor is describedas |Vth|. The power supply line 3922, the potential supply line 3923,and the signal line 3917 are also described as a first wiring, a secondwiring, and a third wiring, respectively. Further, the first scan line3918, the second scan line 3919, the third scan line 3920, and thefourth scan line 3921 may also be described as a fourth wiring, a fifthwiring, a sixth wiring, and a seventh wiring, respectively.

A first electrode (one of a source electrode and a drain electrode) ofthe transistor 3910 is connected to a pixel electrode of thelight-emitting element 3916; a second electrode (the other of the sourceelectrode and the drain electrode) of the transistor 3910 is connectedto the power supply line 3922; and a gate electrode of the transistor3910 is connected to the power supply line 3922 through the fourthswitch 3914 and the second switch 3912. Note that the fourth switch 3914is connected between the gate electrode of the transistor 3910 and thesecond switch 3912. In addition, if a connection point of the fourthswitch 3914 and the second switch 3912 is denoted by a node 3930, thenode 3930 is connected to the signal line 3917 through the first switch3911. Further, the first electrode of the transistor 3910 is alsoconnected to the potential supply line 3923 through the third switch3913.

In addition, the capacitor 3915 is connected between the node 3930 andthe first electrode of the transistor 3910. That is, a first electrodeof the capacitor 3915 is connected to the gate electrode of thetransistor 3910 through the fourth switch 3914, and a second electrodeof the capacitor 3915 is connected to the first electrode of thetransistor 3910. The capacitor 3915 may be formed by sandwiching aninsulating film with a wiring, a semiconductor layer, or an electrode,or can be omitted by using gate capacitance of the transistor in somecases. Such a means which holds a voltage is described as a storagecapacitor. Further, a connection point of the node 3930 and a wiring towhich the first switch 3911 and the first electrode of the capacitor3915 are connected is denoted by a node 3931, and a connection point ofthe first electrode of the transistor 3910 and a wiring to which thesecond electrode of the capacitor 3915 and the pixel electrode of thelight-emitting element 3916 are connected is denoted by a node 3932.

By inputting signals into the first scan line 3918, the second scan line3919, the third scan line 3920, and the fourth scan line 3921, on/off ofthe first switch 3911, the second switch 3912, the third switch 3913,and the fourth switch 3914 is controlled, respectively.

A signal in accordance with a gray scale of the pixel which correspondsto a video signal, that is, a potential in accordance with luminancedata is inputted to the signal line 3917.

Next, operations of the pixel shown in FIG. 39 are described withreference to a timing chart in FIG. 40, and FIGS. 41A to 41D. Note that,in FIG. 40, one frame period which corresponds to a period fordisplaying an image for one screen is divided into an initializationperiod, a threshold voltage writing period, a data writing period, and alight-emitting period. In addition, the initialization period, thethreshold voltage (Vth) writing period, and the data writing period arecollectively described as an address period. Although one frame periodis not particularly limited to a certain period, it is preferable thatone frame period be at least 1/60 second or less so that an image viewerdoes not perceive a flicker.

Note that a potential of V1 is inputted to an opposite electrode 3924 ofthe light-emitting element 3916 and a potential of V1+|Vth|+α (α: anarbitrary positive number) is inputted to the potential supply line3923. In addition, the potential of V1 is inputted to the power supplyline 3922 in the address period, and a potential of V2 is inputted tothe power supply line 3922 in the light-emitting period. Note that V2>V1is satisfied.

Here, although a potential of the opposite electrode 3924 of thelight-emitting element 3916 is equal to a potential of the power supplyline 3922 in the address period for description of operations, thepotential of the opposite electrode 3924 may be any potential as long asit is higher than the sum of a potential of the potential supply line3923 and V_(EL) when a potential difference which is at least necessaryfor the light-emitting element 3916 to emit light is V_(EL). That is, inthe address period, potentials of both ends of the light-emittingelement 3916 may be any potential as long as a current does not flow tothe light-emitting element 3916. In addition, the potential V2 of thepower supply line 3922 in the light-emitting period may be any potentialas long as it is lower than a value which is obtained by subtracting thepotential difference (V_(EL)) which is at least necessary for thelight-emitting element 3916 to emit light from the potential of theopposite electrode 3924; here, since the potential of the oppositeelectrode 3924 is V1 for description, V2 may be any potential lower thanV1−V_(EL).

First, in the initialization period, the first switch 3911 is turned offand the second switch 3912, the third switch 3913, and the fourth switch3914 are turned on as shown (A) in FIG. 40, and in FIG. 41A. At thistime, the first electrode of the transistor 3910 is the sourceelectrode, and a potential thereof is equal to a potential of thepotential supply line 3923 which is V1+|Vth|+α. On the other hand, apotential of the gate electrode of the transistor 3910 is V1. Therefore,the absolute value of a gate-source voltage |Vgs| of the transistor 3910is |Vth|+α so that the transistor 3910 is turned on. Then, |Vth|+α isheld in the capacitor 3915 provided between the gate electrode and thefirst electrode of the transistor 3910. Although the case where thefourth switch 3914 is turned on is described, the fourth switch 3914 maybe turned off. Note that in the following threshold voltage writingperiod, the fourth switch 3914 is required to be turned on.

In the threshold voltage writing period (B) shown in FIG. 40B, and inFIG. 41B, the third switch 3913 is turned off. Therefore, the potentialof the first electrode, that is, the source electrode of the transistor3910 lowers gradually, and when the potential reaches V1+|Vth|, thetransistor 3910 is turned off. Accordingly, a voltage held in thecapacitor 3915 is approximately |Vth|.

In the following data writing period (C) shown in FIG. 40 and in FIG.41C, the first switch 3911 is turned on after the second switch 3912 andthe fourth switch 3914 are turned off, and a potential in accordancewith luminance data (V1−Vdata) is inputted from the signal line 3917. Byturning off the fourth switch 3914 in this period, the transistor 3910can be held to be turned off. Therefore, potential fluctuation of thesecond electrode of the capacitor 3915 caused by a current supplied fromthe power supply line 3922 at the time of data writing can besuppressed. Accordingly, a voltage Vcs which is held in the capacitor3915 at this time can be represented by Formula (5) when electrostaticcapacitance of the capacitor 3915 is C1 and electrostatic capacitance ofthe light-emitting element 3916 is C2.

$\begin{matrix}\lbrack {{Formula}\mspace{14mu} 5} \rbrack & \; \\{{Vcs} = {{{- {{Vth}}} - {{Vdata} \times \frac{C\; 2}{{C\; 1} + {C\; 2}}}}}} & (5)\end{matrix}$

Note that since the light-emitting element 3916 has thinner filmthickness and a larger electrode area than the capacitor 3915, C2>>C1 issatisfied. Therefore, the voltage Vcs which is held in the capacitor3915 is represented by Formula (6) from C2/(C1+C2)≈1. Note also that inthe case where the light-emitting element 3916 is controlled not to emitlight in the following light-emitting period, a potential of Vdata≦0 isinput.[Formula 6]Vcs=|−|Vth|−Vdata|  (6)

Next, in the light-emitting period (D) shown in FIG. 40 and in FIG. 41D,the fourth switch 3914 is turned on after the first switch 3911 isturned off and the potential of the power supply line 3922 is made V2.At this time, the gate-source voltage of the transistor 3910 isVgs=−Vdata−|Vth| so that the transistor 3910 is turned on. Therefore, acurrent in accordance with luminance data flows to the transistor 3910and the light-emitting element 3916, so that the light-emitting element3916 emits light.

Note that a current I flowing to the light-emitting element isrepresented by Formula (7) in the case of operating the transistor 3910in a saturation region.

$\begin{matrix}\lbrack {{Formula}\mspace{14mu} 7} \rbrack & \; \\\begin{matrix}{I = {\frac{1}{2}( \frac{W}{L} )\mu\;{{Cox}( {{Vgs} - {Vth}} )}^{2}}} \\{= {\frac{1}{2}( \frac{W}{L} )\mu\;{{Cox}( {{- {Vdata}} - {{Vth}} - {Vth}} )}^{2}}}\end{matrix} & (7)\end{matrix}$

Since the transistor 3910 is a P-channel transistor, Vth<0 is satisfied.Therefore Formula (7) can be transformed to Formula (8).

$\begin{matrix}\lbrack {{Formula}\mspace{14mu} 8} \rbrack & \; \\{I = {\frac{1}{2}( \frac{W}{L} )\mu\;{{Cox}( {- {Vdata}} )}^{2}}} & (8)\end{matrix}$

In addition, in the case of operating the transistor 3910 in a linearregion, the current I flowing to the light-emitting element isrepresented by Formula (9).

$\begin{matrix}\lbrack {{Formula}\mspace{14mu} 9} \rbrack & \; \\\begin{matrix}{I = {( \frac{W}{L} )\mu\;{{Cox}\lbrack {{( {{Vgs} - {Vth}} ){Vds}} - {\frac{1}{2}{Vds}^{2}}} \rbrack}}} \\{= {( \frac{W}{L} )\mu\;{{Cox}\lbrack {{( {{- {Vdata}} - {{Vth}} - {Vth}} ){Vds}} - {\frac{1}{2}{Vds}^{2}}} \rbrack}}}\end{matrix} & (9)\end{matrix}$

Since Vth<0 is satisfied, Formula (9) can be transformed to Formula(10).

$\begin{matrix}\lbrack {{Formula}\mspace{14mu} 10} \rbrack & \; \\{I = {( \frac{W}{L} )\mu\;{{Cox}\lbrack {{( {- {Vdata}} ){Vds}} - {\frac{1}{2}{Vds}^{2}}} \rbrack}}} & (10)\end{matrix}$

Here, W means channel width of the transistor 3910; L means channellength of the transistor 3910; μ means mobility of the transistor 3910;and Cox means storage capacitance of the transistor 3910.

According to Formula (8) and Formula (10), a current flowing to thelight-emitting element 3916 does not depend on the threshold voltage(Vth) of the transistor 3910 in each of the case where the transistor3910 is operated in the saturation region and the case where thetransistor 3910 is operated in the linear region. Therefore, variationsof a current value caused by variations in the threshold voltage of thetransistor 3910 can be suppressed, so that the current in accordancewith luminance data can be supplied to the light-emitting element 3916.

Accordingly, variations in luminance caused by variations in thethreshold voltage of the transistor 3910 can be suppressed. In addition,since the potential of the opposite electrode 3924 is fixed at aconstant potential, power consumption can be reduced.

Further, in the case of operating the transistor 3910 in the saturationregion, variations in luminance caused by deterioration of thelight-emitting element 3916 can also be reduced. When the light-emittingelement 3916 deteriorates, V_(EL) of the light-emitting element 3916 isincreased and the potential of the first electrode, that is, the sourceelectrode of the transistor 3910 decreases. At this time, the sourceelectrode of the transistor 3910 is connected to the second electrode ofthe capacitor 3915; the gate electrode of the transistor 3910 isconnected to the first electrode of the second electrode of thecapacitor 3915 and is in a floating state. Therefore, a gate potentialof the transistor decreases by the same potential as a potential inaccordance with decrease in the source potential. Accordingly, since Vgsof the transistor 3910 does not change, a current flowing to thetransistor 3910 and the light-emitting element 3916 is not affected evenif the light-emitting element 3916 deteriorates. Note that it can beseen in Formula (8) that the current I flowing to the light-emittingelement 3916 does not depend on the source potential or a drainpotential.

Therefore, in the case of operating the transistor 3910 in thesaturation region, variations in the current flowing to the transistor3910 caused by variations in the threshold voltage of the transistor3910 and deterioration of the light-emitting element 3916 can besuppressed.

Note that in the case of operating the transistor 3910 in the saturationregion, channel length L of the transistor 3910 is preferably long inorder to suppress increase in the amount of current caused by avalanchebreakdown or channel length modulation.

In addition, since a reverse bias voltage is applied to thelight-emitting element 3916 in the initialization period, ashort-circuited portion in the light-emitting element 3916 can beinsulated or deterioration of the light-emitting element 3916 can besuppressed. Therefore, a life of the light-emitting element 3916 can beextended.

Note that the light-emitting element 3916 shown in FIG. 39 is notparticularly limited to a certain type, and an EL element (an organic ELelement, an inorganic EL element, or an EL element including both anorganic material and an inorganic material), an electron-emissiveelement, a liquid crystal element, electronic ink, and the like can beapplied.

Note that it is only necessary for the transistor 3910 to have afunction for controlling a current value supplied to the light-emittingelement 3916, and a type of the transistor 3910 is not limited.Accordingly, a thin film transistor (TFT) using a crystallinesemiconductor film, a thin film transistor using a non-crystallinesemiconductor film typified by amorphous silicon or polycrystallinesilicon, a transistor formed by using a semiconductor substrate or anSOI substrate, a MOS transistor, a junction transistor, a bipolartransistor, a transistor using a compound semiconductor such as ZnO ora-InGaZnO, a transistor using an organic semiconductor or a carbonnanotube, or other transistors can be applied.

The first switch 3911 is a switch which selects timing for inputting asignal in accordance with a gray scale of the pixel from the signal line3917 into the pixel. The second switch 3912 is a switch which selectstiming for supplying a predetermined potential to the gate electrode ofthe transistor 3910 and controls whether to supply the predeterminedpotential to the gate electrode of the transistor 3910. The third switch3913 is a switch which selects timing for supplying a predeterminedpotential for initializing a potential written into the capacitor 3915and raises the potential of the first electrode of the transistor 3910.The fourth switch 3914 is a switch which suppresses the potentialfluctuation of the second electrode of the capacitor 3915 at the time ofdata writing. Therefore, the first switch 3911, the second switch 3912,the third switch 3913, and the fourth switch 3914 are not particularlylimited as long as they have the aforementioned functions. For example,each of the first switch 3911, the second switch 3912, the third switch3913, and the fourth switch 3914 may be a transistor, a diode, or alogic circuit combining them.

Note that, the polarity (a conductivity type) of each transistor is notparticularly limited to a certain type. However, a transistor ofpolarity with small off-current is preferably used. A transistorprovided with an LDD region, a transistor with a multi-gate structure,or the like is given as an example of a transistor with smalleroff-current. In addition, a CMOS switch may be employed by using bothN-channel and P-channel transistors.

For example, in the case where P-channel transistors are employed as thefirst switch 3911, the second switch 3912, the third switch 3913, andthe fourth switch 3914, L-level signals are inputted to scan lines whichcontrol on/off of respective switches in order to turn on the switches,and H-level signals are inputted to the can lines which control on/offof respective switches in order to turn off the switches.

Further, since the pixel can be formed by using only P-channeltransistors, a manufacturing process can be simplified.

In addition, the pixel shown in this embodiment mode can be applied tothe display device in FIG. 9. Similarly to Embodiment Mode 1, unless thedata writing period in each row overlaps, an initialization start periodcan be freely set in each row. Further, since each pixel can emit lightexcept in its address period, a ratio of a light-emitting period in oneframe period (i.e., a duty ratio) can be extremely raised and can alsobe approximately 100%. Accordingly, a display device with few variationsin luminance and a high duty ratio can be obtained.

In addition, since the threshold voltage writing period can also be setlong, the threshold voltage of a transistor which controls a currentvalue flowing to the light-emitting element can be written in thecapacitor more accurately. Therefore, reliability as a display devicecan be improved.

Note that this embodiment mode can be freely combined with any pixelconfiguration shown in another embodiment mode. For example, there arethe case where the fourth switch 3914 is connected between the node 3930and the node 3931 or between the first electrode of the transistor 3910and the node 3932, the case where the second electrode of the transistor3910 is connected to the power supply line 3922 through the fourthswitch 3914, and the like. Note that when a connection point of thepower supply line 3922 and a wiring to which the second switch 3912 andthe second electrode of the transistor 3910 is a node 3935, the fourthswitch 3914 cannot be turned on in the initialization period in the casewhere the fourth switch 3914 is connected between the node 3935 and thepower supply line 3922.

This embodiment mode can be applied to any pixel configuration shown inanother embodiment mode, without limiting to the aforementioneddescription.

Embodiment Mode 7

In this embodiment mode, one mode of a partial sectional view of thepixel of the invention is described with reference to FIG. 17. Note thata transistor shown in the partial sectional view in this embodiment modeis a transistor having a function of controlling a current valuesupplied to a light-emitting element.

First, a base film 1712 is formed over a substrate 1711 having aninsulating surface. As the substrate 1711 having the insulating surface,an insulating substrate such as a glass substrate, a quartz substrate, aplastic substrate (e.g., polyimide, acrylic, polyethylene terephthalate,polycarbonate, polyarylate, or polyethersulfone), or a ceramicsubstrate; or a metal substrate (e.g., tantalum, tungsten, ormolybdenum), a semiconductor substrate, or the like on the surface ofwhich an insulating film is formed, can be used. Note that it isnecessary to use a substrate which can withstand at least heat generatedduring a process.

The base film 1712 is formed of a single layer or a plurality of layersincluding two or more layers of an insulating film such as a siliconoxide film, a silicon nitride film, or a silicon oxynitride(SiO_(x)N_(y)) film. Note that the base film 1712 may be formed bysputtering, CVD, or the like. Although the base film 1712 is a singlelayer in this embodiment mode, it may be a plurality of layers includingtwo or more layers.

Next, a transistor 1713 is formed over the base film 1712. Thetransistor 1713 includes at least a semiconductor layer 1714, a gateinsulating film 1715 formed over the semiconductor layer 1714, and agate electrode 1716 formed over the semiconductor layer 1714 with thegate insulating film 1715 interposed therebetween. The semiconductorlayer 1714 includes a source region and a drain region.

The semiconductor layer 1714 can be formed of a film having anon-crystalline state (i.e., a non-crystalline semiconductor film)selected from an amorphous semiconductor containing silicon, silicongermanium (SiGe), or the like as a main component, as well as amorphoussilicon (a-Si:H), a semi-amorphous semiconductor in which an amorphousstate and a crystalline state are mixed, and a microcrystallinesemiconductor in which crystal grains of 0.5 nm to 20 nm can be observedin an amorphous semiconductor, or a crystalline semiconductor film ofpolysilicon (p-Si:H) or the like. Note that a microcrystalline state inwhich crystal grains of 0.5 nm to 20 nm can be observed is calledmicrocrystal. Note that when a non-crystalline semiconductor film isused for the semiconductor layer 1714, it may be formed by sputtering,CVD, or the like, and when a crystalline semiconductor film is used forthe semiconductor layer 1714, it may be formed by, for example, forminga non-crystalline semiconductor film and then crystallizing it. Ifnecessary, a slight amount of an impurity element (e.g., phosphorus,arsenic, or boron) may be contained in the semiconductor layer 1714 inaddition to the above main component in order to control the thresholdvoltage of a transistor.

Next, a gate insulating film 1715 is formed so as to cover thesemiconductor layer 1714. The gate insulating film 1715 is formed of asingle layer or a plurality of layers using, for example, silicon oxide,silicon nitride, silicon nitride oxide, or the like. CVD, sputtering, orthe like can be used as a film formation method thereof.

Then, a gate electrode 1716 is formed above the semiconductor layer 1714with the gate insulating film 1715 interposed therebetween. The gateelectrode 1716 may be formed of a single layer or may be formed bystacking a plurality of metal films. Note that the gate electrode can beformed of a metal element selected from tantalum (Ta), tungsten (W),titanium (Ti), molybdenum (Mo), aluminum (Al), copper (Cu), and chromium(Cr), or an alloy or compound material containing the element as a maincomponent. For example, the gate electrode may be formed of a firstconductive film using tantalum nitride (TaN) and a second conductivefilm using tungsten (W).

Next, an impurity which imparts N-type or P-type conductivity isselectively added into the semiconductor layer 1714 by using as a maskthe gate electrode 1716 or a resist which is formed into a desiredshape. In this manner, a channel forming region and an impurity region(including a source region, a drain region, a GOLD region, and an LDDregion) are formed in the semiconductor layer 1714. In addition, thetransistor 1713 can be formed as either an N-channel transistor or aP-channel transistor depending on the conductivity type of the impurityelement to be added.

Note that in order to form an LDD region 1720 in a self-aligned mannerin FIG. 17, a silicon compound, such as a silicon oxide film, a siliconnitride film, or a silicon oxynitride film is formed so as to cover thegate electrode 1716, and then is etched back to form a sidewall 1717.After that, the semiconductor layer 1714 is doped with the impuritywhich imparts conductivity, so that a source region 1718, a drain region1719, and an LDD region 1720 can be formed. Therefore, the LDD region1720 is located below the sidewall 1717. Note that the sidewall 1717which is provided to form the LDD region 1720 in a self-aligned manneris not necessarily provided. Note that phosphorus, arsenic, boron, orthe like is used as the impurity which imparts conductivity.

Next, a first interlayer insulating film 1730 is formed by stacking afirst insulating film 1721 and a second insulating film 1722 to coverthe gate electrode 1716. As each of the first insulating film 1721 andthe second insulating film 1722, an inorganic insulating film such as asilicon oxide film, a silicon nitride film, or a silicon oxynitride(SiO_(x)N_(y)) film or an organic resin film (a photosensitive ornon-photosensitive organic resin film) with a low dielectric constantcan be used. Further, a film containing siloxane may also be used. Notethat siloxane is a material in which a skeleton structure is formed bythe bond of silicon (Si) and oxygen (O), and an organic group (e.g., analkyl group or aromatic hydrocarbon) is used as a substituent. Further,a fluoro group may also be contained as a substituent.

Note that insulating films made of the same material may be used as thefirst insulating film 1721 and the second insulating film 1722. In thisembodiment mode, the first interlayer insulating film 1730 has astacked-layer structure of two layers; however, it may be a single layeror have a stacked-layer structure of three or more layers.

Note that the first insulating film 1721 and the second insulating film1722 may be formed by sputtering, CVD, spin coating, or the like, andmay be formed by coating when an organic resin film or a film containingsiloxane is used.

After that, source and drain electrodes 1723 are formed over the firstinterlayer insulating film 1730. Note that the source and drainelectrodes 1723 are connected to the source region 1718 and the drainregion 1719 respectively through contact holes.

Note that each of the source and drain electrodes 1723 can be formed ofa metal such as silver (Ag), gold (Au), copper (Cu), nickel (Ni),platinum (Pt), palladium (Pd), iridium (Ir), rhodium (Rh), tungsten (W),aluminum (Al), tantalum (Ta), molybdenum (Mo), cadmium (Cd), zinc (Zn),iron (Fe), titanium (Ti), silicon (Si), germanium (Ge), zirconium (Zr),or barium (Ba), an alloy thereof, metal nitride thereof, or astacked-layer film thereof.

Next, a second interlayer insulating film 1731 is formed so as to coverthe source and drain electrodes 1723. As the second interlayerinsulating film 1731, an inorganic insulating film, a resin film, or astacked layer thereof can be used. As the inorganic insulating film, asilicon nitride film, a silicon oxide film, a silicon oxynitride film,or a stacked-layer film thereof can be used. For the resin film,polyimide, polyamide, acrylic, polyimide amide, epoxy, or the like canbe used.

A pixel electrode 1724 is formed over the second interlayer insulatingfilm 1731. Next, an insulator 1725 is formed so as to cover an endportion of the pixel electrode 1724. The insulator 1725 is preferablyformed to have a curved surface with curvature at an upper end or alower end thereof in order to favorably form a layer 1726 containing alight-emitting substance later. For example, when positivephotosensitive acrylic is used as a material of the insulator 1725, theinsulator 1725 is preferably formed to have a curved surface with acurvature radius (0.2 μm to 3 μm) only at the upper end thereof. Eithera negative photosensitive material which becomes insoluble in an etchantby light irradiation or a positive photosensitive material which becomessoluble in an etchant by light irradiation can be used for the insulator1725. Further, an inorganic material such as silicon oxide or siliconoxynitride as well as an organic material can be used as a material ofthe insulator 1725.

Next, a layer 1726 containing a light-emitting substance and an oppositeelectrode 1727 are formed over the pixel electrode 1724 and theinsulator 1725.

Note that a light-emitting element 1728 is formed in a region where thelayer 1726 containing a light-emitting substance is sandwiched betweenthe pixel electrode 1724 and the opposite electrode 1727.

Next, the detail of the light-emitting element 1728 is described withreference to FIGS. 18A and 18B. Note that the pixel electrode 1724 andthe opposite electrode 1727 in FIG. 17 correspond to a pixel electrode1801 and an opposite electrode 1802 in FIGS. 18A and 18B. In FIG. 18A,the pixel electrode is an anode and the opposite electrode is a cathode.

As shown in FIG. 18A, a hole injection layer 1811, a hole transportlayer 1812, an electron transport layer 1814, an electron injectionlayer 1815, and the like are provided in addition to a light-emittinglayer 1813 between the pixel electrode 1801 and the opposite electrode1802. These layers are stacked so that holes are injected from the pixelelectrode 1801 side and electrons are injected from the oppositeelectrode 1802 side when a voltage is applied such that a potential ofthe pixel electrode 1801 is higher than that of the opposite electrode1802.

In such a light-emitting element, the holes injected from the pixelelectrode 1801 and the electrons injected from the opposite electrode1802 are recombined in the light-emitting layer 1813 to excite thelight-emitting substance. Then, light emission occurs when the excitedlight-emitting substance returns to a ground state. Note that anysubstance which can provide luminescence (electroluminescence) can beused as the light-emitting substance.

There is no particular limitation on the substance forming thelight-emitting layer 1813, and the light-emitting layer may be formed ofonly a light-emitting substance. However, when concentration quenchingoccurs, the light-emitting layer is preferably a layer in which alight-emitting substance is mixed so as to be dispersed into a layer ofa substance (host) having a larger energy gap than the light-emittingsubstance, thereby preventing concentration quenching of thelight-emitting substance. Note that the energy gap refers to an energydifference between the lowest unoccupied molecular orbital (LUMO) leveland the highest occupied molecular orbital (HOMO) level.

In addition, there is no particular limitation on the light-emittingsubstance, and any substance which can emit light with a desiredemission wavelength may be used. For example, in order to obtain redlight emission, a substance which exhibits light emission having a peakof an emission spectrum at 600 nm to 680 nm can be used, such as4-dicyanomethylene-2-isopropyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbr.: DCJTI),4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbr.: DCJT),4-dicyanomethylene-2-tert-butyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbr.: DCJTB), periflanthene, or2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]benzene.In order to obtain green light emission, a substance which exhibitslight emission having a peak of an emission spectrum at 500 nm to 550 nmcan be used, such as N,N′-dimethylquinacridone (abbr.: DMQd), coumarin6, coumarin 545T, tris(8-quinolinolato)aluminum (abbr.: Alq), orN,N′-diphenylquinacridon (DPQd). In order to obtain blue light emission,a substance which exhibits light emission having a peak of an emissionspectrum at 420 nm to 500 nm can be used, such as9,10-bis(2-naphthyl)-tert-butylanthracene (abbr.: t-BuDNA),9,9′-bianthryl, 9,10-diphenylanthracene (abbr.: DPA),9,10-bis(2-naphthyl)anthracene (abbr.: DNA),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-gallium (abbr.: BGaq),or bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbr.:BAlq).

There is also no particular limitation on the substance which is usedfor dispersing the light-emitting substance, and for example, ananthracene derivative such as 9,10-di(2-naphthyl)-2-tert-butylanthracene(abbr.: t-BuDNA), a carbazole derivative such as4,4′-bis(N-carbazolyl)biphenyl (abbr.: CBP), a metal complex such asbis[2-(2-hydroxyphenyl)pyridinato]zinc (abbr.: Znpp₂) orbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbr.: ZnBOX), or the likecan be used.

Although an anode material forming the pixel electrode 1801 is notparticularly limited, it is preferable to use a metal, an alloy, anelectrically conductive compound, a mixture thereof, or the like havinga high work function (a work function of 4.0 eV or higher). As aspecific example of such an anode material, oxide of a metal materialsuch as indium tin oxide (abbr.: ITO), ITO containing silicon oxide(abbr.: ITSO), or indium zinc oxide (abbr.: IZO) formed by using atarget in which indium oxide is mixed with zinc oxide (ZnO) at 2 wt % to20 wt % can be given. Further, gold (Au), platinum (Pt), nickel (Ni),tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co),copper (Cu), palladium (Pd), nitride of a metal material (e.g., TiN), orthe like can be given.

On the other hand, as a substance forming the opposite electrode 1802, ametal, an alloy, a conductive compound, a mixture thereof, or the likehaving a low work function (a work function of 3.8 eV or less) can beused. As a specific example of such a cathode material, an elementbelonging to Group 1 or 2 of the periodic table, that is, an alkalimetal such as lithium (Li) or cesium (Cs), an alkaline earth metal suchas magnesium (Mg), calcium (Ca), or strontium (Sr), or an alloycontaining them (Mg:Ag, Al:Li) can be given. In addition, by providing alayer having an excellent electron injection property between theopposite electrode 1802 and the light-emitting layer 1813 so as to bestacked with the opposite electrode, various conductive materialsincluding the materials described as the material of the pixel electrode1801 such as Al, Ag, ITO, or ITO containing silicon oxide can be usedfor the opposite electrode 1802 regardless of the value of the workfunction. Further, a similar effect can be obtained by using a materialparticularly having an excellent electron injecting function for formingthe electron injection layer 1815 described later.

Note that in order to extract light emission to outside, it ispreferable that one or both of the pixel electrode 1801 and the oppositeelectrode 1802 be a transparent electrode made of ITO or the like or anelectrode formed with a thickness of several to several tens nm so as tobe able to transmit visible light.

The hole transport layer 1812 is provided between the pixel electrode1801 and the light-emitting layer 1813 as shown in FIG. 18A. The holetransport layer is a layer having a function of transporting holesinjected from the pixel electrode 1801 to the light-emitting layer 1813.By providing the hole transport layer 1812 to separate the pixelelectrode 1801 and the light-emitting layer 1813 from each other asdescribed above, light emission can be prevented from being quenched dueto metal.

Note that the hole transport layer 1812 is preferably formed using asubstance having a high hole transport property, and in particular, asubstance having a hole mobility of 1×10⁻⁶ cm²/Vs or more is preferablyused. Note that the substance having a high hole transport propertyrefers to a substance having a higher mobility of holes than electrons.As specific examples of a substance capable of forming the holetransport layer 1812, there are4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbr.: NPB),4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbr.: TPD),4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbr.: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbr.:MTDATA),4,4′-bis{N-[4-(N,N-di-m-tolylamino)phenyl]-N-phenylamino}biphenyl(abbr.: DNTPD), 1,3,5-tris[N,N-di(m-tolyl)amino]benzene (abbr.:m-MTDAB), 4,4′,4″-tris(N-carbazolyl)tripheylamine (abbr.: TCTA),phthalocyanine (abbr.: H₂Pc), copper phthalocyanine (abbr.: CuPc),vanadyl phthalocyanine (abbr.: VOPc), and the like. In addition, thehole transport layer 1812 may be a layer having a multi-layer structurewhich is formed by combining two or more layers formed of any of theaforementioned substances.

Further, the electron transport layer 1814 may be provided between theopposite electrode 1802 and the light-emitting layer 1813 as shown inFIG. 18A. Here, the electron transport layer is a layer having afunction of transporting electrons injected from the opposite electrode1802 to the light-emitting layer 1813. By providing the electrontransport layer 1814 to separate the opposite electrode 1802 and thelight-emitting layer 1813 from each other as described above, lightemission can be prevented from being quenched due to metal of theelectrode material.

There is no particular limitation on the material of the electrontransport layer 1814, and the electron transport layer 1814 can beformed using a metal complex having a quinoline skeleton or abenzoquinoline skeleton such as tris(8-quinolinolato)aluminum (abbr.:Alq), tris(4-methyl-8-quinolinolato)aluminum (abbr.: Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbr.: BeBq₂), Orbis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbr.: BAlq),or the like. Further, the electron transport layer 1814 may also beformed using a metal complex having an oxazole ligand or a thiazoleligand such as bis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (abbr.:Zn(BOX)₂) or bis[2-(2-hydroxypheny 1)-benzothiazolato]zinc (abbr.:Zn(BTZ)₂), or the like. Further alternatively, the electron transportlayer 1814 may be formed using2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbr.: PBD),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbr.:OXD-7), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbr.: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbr.: p-EtTAZ), bathophenanthroline (abbr.: BPhen), bathocuproin(abbr.: BCP), or the like. The electron transport layer 1814 ispreferably formed using a substance having a higher mobility ofelectrons than holes as described above. In addition, the electrontransport layer 1814 is preferably formed using a substance having anelectron mobility of 1×10⁻⁶ cm²/Vs or more. Note that the electrontransport layer 1814 may have a multi-layer structure which is formed bycombining two or more layers formed of any of the aforementionedsubstances.

Further, the hole injection layer 1811 may be provided between the pixelelectrode 1801 and the hole transport layer 1812 as shown in FIG. 18A.Here, the hole injection layer refers to a layer having a function ofpromoting hole injection from the electrode functioning as the anode tothe hole transport layer 1812.

There is no particular limitation on the material of the hole injectionlayer 1811, and the hole injection layer 1811 can be formed using metaloxide such as molybdenum oxide (MoOx), vanadium oxide (VOx), rutheniumoxide (RuOx), tungsten oxide (WOx), or manganese oxide (MnOx). Further,the hole injection layer 1811 can also be formed using aphthalocyanine-based compound such as phthalocyanine (abbr.: H₂Pc) orcopper phthalocyanine (CuPc), an aromatic amine-based compound such as4,4-bis(N-(4-(N,N-di-m-tolylamino)phenyl)-N-phenylamino)biphenyl (abbr.:DNTPD), a high molecule such as a poly(ethylenedioxythiophene)/poly(styrenesulfonic acid) aqueous solution (PEDOT/PSS),or the like.

Further, a mixture of the aforementioned metal oxide and a substancehaving a high hole transport property may be provided between the pixelelectrode 1801 and the hole transport layer 1812. Such a layer does notcause a rise in driving voltage even when it is thickened; therefore,optical design using a microcavity effect or a light interference effectcan be conducted by adjusting the thickness of the layer. Therefore, ahigh-quality light-emitting element with excellent color purity and fewchanges in color that are dependent on viewing angles can bemanufactured. In addition, the film thickness of such a layer can becontrolled so as to prevent short circuit between the pixel electrode1801 and the opposite electrode 1802 that would occur due toirregularities generated at the film formation on the surface of thepixel electrode 1801 or due to minute residues remaining on theelectrode surface.

Further, the electron injection layer 1815 may be provided between theopposite electrode 1802 and the electron transport layer 1814 as shownin FIG. 18A. Here, the electron injection layer is a layer having afunction of promoting electron injection from the electrode functioningas the cathode to the electron transport layer 1814. Note that when theelectron transport layer is not particularly provided, electroninjection to the light-emitting layer may be helped by providing theelectron injection layer between the electrode functioning as thecathode and the light-emitting layer.

There is no particular limitation on the material of the electroninjection layer 1815, and the electron injection layer 1815 can beformed using a compound of alkali metal or alkaline earth metal, such aslithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride(CaF₂). Further, the electron injection layer 1815 can also be formedusing a mixture of a substance having a high electron transport propertysuch as Alq or 4,4-bis(5-methylbenzoxazol-2-yl)stilbene (BzOs), andalkali metal or alkaline earth metal such as magnesium or lithium.

Note that each of the hole injection layer 1811, the hole transportlayer 1812, the light-emitting layer 1813, the electron transport layer1814, and the electron injection layer 1815 may be formed by any of anevaporation method, an ink-jet method, a coating method, and the like.In addition, each of the pixel electrode 1801 and the opposite electrode1802 may be formed by any of a sputtering method, an evaporation method,and the like.

The layer structure of the light-emitting element is not limited to theone shown in FIG. 18A; the light-emitting element may be formedsequentially from an electrode functioning as a cathode as shown in FIG.18B. That is, the pixel electrode 1801 may be formed as a cathode, andthen the electron injection layer 1815, the electron transport layer1814, the light-emitting layer 1813, the hole transport layer 1812, thehole injection layer 1811, and the opposite electrode 1802 may bestacked sequentially over the pixel electrode 1801. Note that theopposite electrode 1802 functions as an anode.

Although the light-emitting element having a single light-emitting layeris described here, the light-emitting element may include a plurality oflight-emitting layers. By providing a plurality of light-emitting layersso that light emissions from the light-emitting layers are mixed, whitelight can be obtained. For example, in the case of a light-emittingelement including two light-emitting layers, it is preferable to providea spacing layer, or a layer which generates holes and a layer whichgenerates electrons between a first light-emitting layer and a secondlight-emitting layer. By employing this structure, the light emitted tooutside is visually mixed and perceived as white light; thus, whitelight can be obtained.

Light emission is extracted to outside through one or both of the pixelelectrode 1724 and the opposite electrode 1727 in FIG. 17. Therefore,one or both of the pixel electrode 1724 and the opposite electrode 1727is/are formed of a light-transmitting substance.

When only the opposite electrode 1727 is formed of a light-transmittingsubstance, light emission is extracted from a side opposite to thesubstrate through the opposite electrode 1727 as shown in FIG. 19A. Whenonly the pixel electrode 1724 is formed of a light-transmittingsubstance, light emission is extracted from the substrate side throughthe pixel electrode 1724 as shown in FIG. 19B. When both of the pixelelectrode 1724 and the opposite electrode 1727 are formed oflight-transmitting substances, light emission is extracted from both ofthe substrate side and the opposite side thereof through the pixelelectrode 1724 and the opposite electrode 1727 as shown in FIG. 19C.

The material of each wiring or electrode is not limited to theabove-described materials, and one element or a plurality of elementsselected from aluminum (Al), tantalum (Ta), titanium (Ti), molybdenum(Mo), tungsten (W), neodymium (Nd), chromium (Cr), nickel (Ni), platinum(Pt), gold (Au), silver (Ag), copper (Cu), magnesium (Mg), scandium(Sc), cobalt (Co), zinc (Zn), niobium (Nb), silicon (Si), phosphorus(P), boron (B), arsenic (As), gallium (Ga), indium (In), and tin (Sn), acompound or an alloy material containing one element or a plurality ofelements selected from the above ones (e.g., Indium Tin Oxide (ITO),Indium Zinc Oxide (IZO), ITO containing silicon oxide (ITSO), zinc oxide(ZnO), aluminum-neodymium (Al—Nd), or magnesium-silver (Mg—Ag)), asubstance combining any of the above-described compounds, or the likecan be used. Further, a compound of silicon and any of theabove-described ones (silicide) (e.g., aluminum-silicon,molybdenum-silicon, or nickel silicide) or a compound of nitrogen (e.g.,titanium nitride, tantalum nitride, or molybdenum nitride) can also beused. Note that the silicon (Si) may contain an N-type impurity (e.g.,phosphorus) or a P-type impurity (e.g., boron) at a high concentration;by containing the impurity, the conductivity is improved such that thesimilar action to a general conductor is performed, thereby utilizingthe silicon as a wiring or an electrode more easily. Note that any ofsingle crystalline silicon, polycrystalline silicon (polysilicon), andamorphous silicon can be used as the silicon; the resistance can bereduced in the case of using single crystalline silicon orpolycrystalline silicon, and it becomes possible to manufacture througha simple manufacturing process in the case of using amorphous silicon.

Further, in the case of using aluminum or silver, signal delay can bereduced because of its high conductivity. In addition, since it is easyto be etched, patterning can be easily performed and microfabricationcan be performed. Further, also in the case of using copper, signaldelay can be reduced because of its high conductivity. In the case ofusing molybdenum, a problem such as a material defect does not occur inthe manufacturing process even if molybdenum is in contact with an oxidesemiconductor such as ITO or IZO, or silicon. In addition, patterning oretching can be performed easily and the heat resistance is high. In thecase of using titanium also, a problem such as a material defect doesnot occur in the manufacturing process even if titanium is in contactwith an oxide semiconductor such as ITO or IZO or silicon, and the heatresistance is high. Further, tungsten or neodymium is also preferablebecause of its high heat resistance. Note that when neodymium iscombined with aluminum to be an alloy, the heat resistance is improvedand a hillock of aluminum can be suppressed. Further, silicon can beformed at the same time as a semiconductor layer included in atransistor, and has a high heat resistance. Further, Indium Tin Oxide(ITO), Indium Zinc Oxide (IZO), ITO containing silicon oxide (ITSO),zinc oxide (ZnO), or silicon (Si) each having a light-transmittingproperty is particularly preferable when it is used for a portion thoughwhich light is transmitted; for example, they can be used for a pixelelectrode or a common electrode.

Note that the wiring or the electrode may have a single layer structureor a multi-layer structure formed using any of the above-describedmaterials. For example, in the case of employing a single layerstructure, the manufacturing process can be simplified and cost can bereduced. In the case of employing a multi-layer structure, advantages ofthe materials can be utilized while disadvantages thereof can bedecreased, thereby a wiring or an electrode which is superior inperformance can be formed. For example, by containing a low-resistancematerial (e.g., aluminum) in the multi-layer structure, the resistanceof the wiring can be reduced. Further, by containing a high heatresistance material in the multi-layer structure (e.g., a stacked-layerstructure in which a low heat resistance material having an advantage issandwiched using a high heat resistance material), the heat resistancecan be improved and an advantage which is not utilized in a single layercan be utilized. For example, it is preferable to use a wiring or anelectrode having a structure in which a layer containing aluminum issandwiched using a layer containing molybdenum or titanium. Note thatwhen a wiring or an electrode has a portion which is directly in contactwith a wiring or an electrode formed of another material, they may havean adverse effect on each other. For example, one material is mixed intothe other material to change properties of both the materials, thereby,for example, an original purpose cannot be achieved or a problem occursat the time of manufacturing so that normal manufacturing cannot beperformed. In this case, such a problem can be solved by sandwiching orcovering one layer by another layer. For example, when Indium Tin Oxide(ITO) and aluminum are in contact with each other, titanium ormolybdenum is preferably sandwiched therebwteen. Similarly, also whensilicon and aluminum are made to be in contact with each other, titaniumor molybdenum is preferably sandwiched therebwteen.

Next, a transistor having a staggered structure using a non-crystallinesemiconductor film for a semiconductor layer of the transistor 1713 isdescribed. Partial sectional views of a pixel are shown in FIGS. 20A and20B. Note that in each of FIGS. 20A and 20B, in addition to a transistorhaving a staggered structure, a capacitor included in a pixel isdescribed.

As shown in FIGS. 20A and 20B, a base film 2012 is formed over asubstrate 2011. Further, a pixel electrode 2013 is formed over the basefilm 2012. In addition, a first electrode 2014 is formed of the samematerial in the same layer as the pixel electrode 2013.

Further, a wiring 2015 and a wiring 2016 are formed over the base film2012, and an end portion of the pixel electrode 2013 is covered with thewiring 2015. An N-type semiconductor layer 2017 and an N-typesemiconductor layer 2018 each having N-type conductivity are formed overthe wiring 2015 and the wiring 2016. In addition, a semiconductor layer2019 is formed over the base film 2012 between the wiring 2015 and thewiring 2016. A part of the semiconductor layer 2019 is extended so as tooverlap with the N-type semiconductor layer 2017 and the N-typesemiconductor layer 2018. Note that this semiconductor layer is formedof a non-crystalline semiconductor film made of an amorphoussemiconductor such as amorphous silicon (a-Si:H), a semi-amorphoussemiconductor, a microcrystalline semiconductor, or the like. Inaddition, a gate insulating film 2020 is formed over the semiconductorlayer 2019. An insulating film 2021 made of the same material in thesame layer as the gate insulating film 2020 is also formed over thefirst electrode 2014.

Furthermore, a gate electrode 2022 is formed over the gate insulatingfilm 2020; thus, a transistor 2025 is formed. In addition, a secondelectrode 2023 made of the same material in the same layer as the gateelectrode 2022 is formed over the first electrode 2014 with theinsulating film 2021 interposed therebetween, and a capacitor 2024 isformed in which the insulating film 2021 is sandwiched between the firstelectrode 2014 and the second electrode 2023. An interlayer insulatingfilm 2026 is formed so as to cover the end portion of the pixelelectrode 2013, the transistor 2025, and the capacitor 2024.

A layer 2027 containing a light-emitting substance and an oppositeelectrode 2028 are formed over the interlayer insulating film 2026 andthe pixel electrode 2013 located in an opening of the interlayerinsulating film 2026, and a light-emitting element 2029 is formed in aregion where the layer 2027 containing a light-emitting substance issandwiched between the pixel electrode 2013 and the opposite electrode2028.

The first electrode 2014 shown in FIG. 20A may be formed of the samematerial in the same layer as the wirings 2015 and 2016 as shown in FIG.20B, and a capacitor 2031 may be formed in which the insulating film2021 is sandwiched between a first electrode 2030 and the secondelectrode 2023. Although an N-channel transistor is used as thetransistor 2025 in FIGS. 20A and 20B, a P-channel transistor may also beused.

Materials of the substrate 2011, the base film 2012, the pixel electrode2013, the gate insulating film 2020, the gate electrode 2022, theinterlayer insulating film 2026, the layer 2027 containing alight-emitting substance, and the opposite electrode 2028 may be similarto those of the substrate 1711, the base film 1712, the pixel electrode1724, the gate insulating film 1715, the gate electrode 1716, theinterlayer insulating films 1730 and 1731, the layer 1726 containing alight-emitting substance, and the opposite electrode 1727 shown in FIG.17, respectively. The wirings 2015 and 2016 may be formed by using amaterial similar to those of the source and drain electrodes 1723 inFIG. 17.

Next, partial sectional views of a pixel having a transistor with astructure in which a gate electrode is sandwiched between a substrateand a semiconductor layer, namely a bottom-gate transistor in which agate electrode is located below a semiconductor layer are FIGS. 21A and21B, as another structure of a transistor using a non-crystallinesemiconductor film as a semiconductor layer.

A base film 2112 is formed over a substrate 2111. A gate electrode 2113is formed over the base film 2112. In addition, a first electrode 2114is formed of the same material in the same layer as the gate electrode2113. As a material of the gate electrode 2113, polycrystalline siliconto which phosphorus is added or silicide that is a compound of metal andsilicon may be used as well as the material used for the gate electrode1716 shown in FIG. 17.

A gate insulating film 2115 is formed so as to cover the gate electrode2113 and the first electrode 2114.

A semiconductor layer 2116 is formed over the gate insulating film 2115.A semiconductor layer 2117 made of the same material in the same layeras the semiconductor layer 2116 is formed over the first electrode 2114.Note that this semiconductor layer is formed of a non-crystallinesemiconductor film of an amorphous semiconductor such as amorphoussilicon (a-Si:H), a semi-amorphous semiconductor, a microcrystallinesemiconductor, or the like.

An N-type semiconductor layer 2118 and an N-type semiconductor layer2119 having N-type conductivity are formed over the semiconductor layer2116, and an N-type semiconductor layer 2120 is formed over thesemiconductor layer 2117.

A wiring 2121 and a wiring 2122 are formed over the N-type semiconductorlayer 2118 and the N-type semiconductor layer 2119; thus a transistor2129 is formed. A conductive layer 2123 made of the same material in thesame layer as the wiring 2121 and the wiring 2122 is formed over theN-type semiconductor layer 2120; thus a second electrode includes theconductive layer 2123, the N-type semiconductor layer 2120, and thesemiconductor layer 2117. Note that a capacitor 2130 is formed with astructure in which the gate insulating film 2115 is sandwiched betweenthe second electrode and the first electrode 2114.

One end of the wiring 2121 is extended, and a pixel electrode 2124 isformed in contact with the top portion of the extended wiring 2121.

An insulator 2125 is formed so as to cover an end portion of the pixelelectrode 2124, the transistor 2129, and the capacitor 2130.

A layer 2126 containing a light-emitting substance and an oppositeelectrode 2127 are formed over the pixel electrode 2124 and theinsulator 2125, and a light-emitting element 2128 is formed in a regionwhere the layer 2126 containing a light-emitting substance is sandwichedbetween the pixel electrode 2124 and the opposite electrode 2127.

The semiconductor layer 2117 and the N-type semiconductor layer 2120which serve as a part of the second electrode of the capacitor 2130 donot particularly need to be provided. In other words, a capacitor may beformed with a structure in which the conductive layer 2123 is used asthe second electrode and the gate insulating film 2115 is sandwichedbetween the first electrode 2114 and the conductive layer 2123.

Although an N-channel transistor is used as the transistor 2129, aP-channel transistor may also be used.

Note that by forming the pixel electrode 2124 before the wiring 2121 isformed in FIG. 21A, a capacitor 2132 having a structure in which thegate insulating film 2115 is sandwiched between the first electrode 2114and a second electrode 2131 made of the same material in the same layeras the pixel electrode 2124 can also be formed as shown in FIG. 21B.

Although the description is made on a channel-etch type invertedstaggered transistor, a channel protection type transistor may also beformed of course. Next, the case of a channel protection type transistoris described with reference to FIGS. 22A and 22B. Note that the samereference numerals are used in FIGS. 22A and 22B to denote the sameportions as those in FIGS. 21A and 21B.

A channel protection type transistor 2201 shown in FIG. 22A is differentfrom the channel-etch type transistor 2129 shown in FIG. 21A in that aninsulator 2202 serving as an etching mask is provided over a region forforming a channel in the semiconductor layer 2116.

Similarly, the channel protection type transistor 2201 shown in FIG. 22Bis different from the channel-etch type transistor 2129 shown in FIG.21B in that the insulator 2202 serving as an etching mask is providedover a region for forming a channel in the semiconductor layer 2116.

Manufacturing cost can be reduced by using a non-cyrstallinesemiconductor film for a semiconductor layer of a transistor included inthe pixel of the invention. Note that the materials described withreference to FIG. 17 can be used as respective materials.

Further, structures of a transistor and a capacitor are not limited tothose described above, and transistors and capacitors having variousstructures can be used.

Further, a crystalline semiconductor film of polysilicon (p-Si:H) or thelike may also be used for a semiconductor layer of a transistor, as wellas a non-crystalline semiconductor film of an amorphous semiconductorsuch as amorphous silicon (a-Si:H), a semi-amorphous semiconductor, amicrocrystalline semiconductor, or the like.

FIG. 23 is a partial sectional view of a pixel including a transistorusing a crystalline semiconductor film for a semiconductor layer, and isdescribed below. Note that a transistor 2318 shown in FIG. 23 is themulti-gate transistor shown in FIG. 29.

As shown in FIG. 23, a base film 2302 is formed over a substrate 2301,and a semiconductor layer 2303 is formed thereover. Note that thesemiconductor layer 2303 is formed by patterning a crystallinesemiconductor film into a desired shape.

An example of a manufacturing method of the crystalline semiconductorfilm is described below. First, an amorphous silicon film is formed overthe substrate 2301 by sputtering, CVD, or the like. A film formationmaterial does not need to be limited to an amorphous silicon film aslong as it is a non-crystalline semiconductor film of an amorphoussemiconductor, a semi-amorphous semiconductor, a microcrystallinesemiconductor, or the like. Further, a compound semiconductor filmhaving an amorphous structure such as an amorphous silicon germaniumfilm may also be used.

Then, the formed amorphous silicon film is crystallized using a thermalcrystallization method, a laser crystallization method, a thermalcrystallization method using a catalytic element such as nickel, or thelike, thereby obtaining a crystalline semiconductor film. Note thatcrystallization may also be performed by a combination of thesecrystallization methods.

In the case of forming the crystalline semiconductor film by a thermalcrystallization method, a heating furnace, laser irradiation, RTA (RapidThermal Annealing), or a combination thereof can be used.

In the case of forming the crystalline semiconductor film by a lasercrystallization method, a continuous wave laser beam (a CW laser beam)or a pulsed laser beam can be used. As a laser beam that can be usedhere, a laser beam emitted from one or more kinds of the following canbe used: a gas laser such as an Ar laser, a Kr laser, or an excimerlaser; a laser using, as a medium, single-crystal YAG, YVO₄, forsterite(Mg₂SiO₄), YAlO₃, or GdVO₄, or polycrystalline (ceramic) YAG, Y₂O₃,YVO₄, YAlO₃, or GdVO₄ doped with one or more of Nd, Yb, Cr, Ti, Ho, Er,Tm, and Ta as a dopant; a glass laser; a ruby laser; an alexandritelaser; a Ti:sapphire laser; a copper vapor laser; and a gold vaporlaser. A crystal having a large grain diameter can be obtained byirradiation with the fundamental wave of the above laser beam or thesecond harmonic to the fourth harmonic of the laser beam. For example,the second harmonic (532 nm) or the third harmonic (355 nm) of a Nd:YVO₄laser (the fundamental wave: 1064 nm) can be used. At this time, theenergy density of the laser is required to be approximately 0.01 MW/cm²to 100 MW/cm² (preferably, 0.1 MW/cm² to 10 MW/cm²). The scanning rateis set to approximately 10 cm/sec to 2000 cm/sec for irradiation.

Note that continuous wave oscillation can be performed with a laserusing, as a medium, single-crystal YAG, YVO₄, forsterite (Mg₂SiO₄),YAlO₃, or GdVO₄, or polycrystalline (ceramic) YAG, Y₂O₃, YVO₄, YAlO₃, orGdVO₄ doped with one or more of Nd, Yb, Cr, Ti, Ho, Er, Tm, and Ta as adopant; an Ar ion laser; or a Ti:sapphire laser. Further, it can bepulsed at a repetition rate of 10 MHz or more by performing Q-switchoperation, mode locking, or the like. When the laser beam is pulsed at arepetition rate of 10 MHz or more, the semiconductor film is irradiatedwith the next pulsed laser after being melted by the preceding laserbefore being solidified. Therefore, unlike the case of using a pulsedlaser having a low repetition rate, the interface between solid phaseand liquid phase can be moved continuously in the semiconductor film, sothat crystal grains grown continuously in the scanning direction can beobtained.

In the case of forming a crystalline semiconductor film by a thermalcrystallization method using a catalytic element such as nickel, it ispreferable to perform gettering treatment for removing the catalyticelement such as nickel after the crystallization.

By the above-described crystallization, a crystallized region is formedpartially in the amorphous semiconductor film. This partly crystallizedcrystalline semiconductor film is patterned into a desired shape,thereby forming an island-shaped semiconductor film. This semiconductorfilm is used for the semiconductor layer 2303 of the transistor.

The crystalline semiconductor layer is used for a channel forming region2304 and an impurity region 2305 serving as a source region or a drainregion of the transistor 2318 and also for a semiconductor layer 2306and an impurity region 2308 serving as a lower electrode of a capacitor2319. Note that the impurity region 2308 does not particularly need tobe provided. Channel doping may be performed to the channel formingregion 2304 and the semiconductor layer 2306.

Next, a gate insulating film 2309 is formed over the semiconductor layer2303 and the lower electrode of the capacitor 2319. Further, a gateelectrode 2310 is formed over the semiconductor layer 2303 with the gateinsulating film 2309 interposed therebetween, and an upper electrode2311 made of the same material in the same layer as the gate electrode2310 is formed over the semiconductor layer 2306 of the capacitor 2319with the gate insulating film 2309 interposed therebetween. In thismanner, the transistor 2318 and the capacitor 2319 are manufactured.

Next, an interlayer insulating film 2312 is formed so as to cover thetransistor 2318 and the capacitor 2319, and a wiring 2313 is formed overthe interlayer insulating film 2312 so as to be in contact with theimpurity region 2305 through a contact hole. Then, a pixel electrode2314 is formed in contact with the wiring 2313 over the interlayerinsulating film 2312, and an insulator 2315 is formed so as to cover anend portion of the pixel electrode 2314 and the wiring 2313. Further, alayer 2316 containing a light-emitting substance and an oppositeelectrode 2317 are formed over the pixel electrode 2314, and alight-emitting element 2320 is formed in a region where the layer 2316containing a light-emitting substance is sandwiched between the pixelelectrode 2314 and the opposite electrode 2317.

A partial cross section of a pixel including a bottom-gate transistorusing a crystalline semiconductor film of polysilicon (p-Si:H) or thelike for a semiconductor layer is shown in FIG. 24.

A base film 2402 is formed over a substrate 2401, and a gate electrode2403 is formed thereover. In addition, a first electrode 2404 of acapacitor 2423 is formed of the same material in the same layer as thegate electrode 2403.

A gate insulating film 2405 is formed so as to cover the gate electrode2403 and the first electrode 2404.

A semiconductor layer is formed over the gate insulating film 2405. Notethat the semiconductor layer is formed by crystallizing anon-crystalline semiconductor film of an amorphous semiconductor, asemi-amorphous semiconductor, a microcrystalline semiconductor, or thelike by using a thermal crystallization method, a laser crystallizationmethod, a thermal crystallization method using a catalytic element suchas nickel, or the like and patterning into a desired shape.

Note that the channel forming region 2406, an LDD region 2407, and animpurity region 2408 serving as a source region or a drain region of atransistor 2422, and a region 2409 serving as a second electrode, andimpurity regions 2410 and 2411 of the capacitor 2423 are formed usingthe semiconductor layer. Note that the impurity regions 2410 and 2411are not particularly required to be provided. In addition, the channelforming region 2406 and the region 2409 may be doped with an impurity.

Note that the capacitor 2423 has a structure in which the gateinsulating film 2405 is sandwiched between the first electrode 2404 andthe second electrode including the region 2409 and the like formed ofthe semiconductor layer.

Next, a first interlayer insulating film 2412 is formed so as to coverthe semiconductor layer, and a wiring 2413 is formed over the firstinterlayer insulating film 2412 so as to be in contact with the impurityregion 2408 through a contact hole.

An opening 2415 is formed in the first interlayer insulating film 2412.A second interlayer insulating film 2416 is formed so as to cover thetransistor 2422, the capacitor 2423, and the opening 2415, and a pixelelectrode 2417 is formed over the second interlayer insulating film 2416so as to be connected to the wiring 2413 through a contact hole. Inaddition, an insulator 2418 is formed so as to cover an end portion ofthe pixel electrode 2417. Then, a layer 2419 containing a light-emittingsubstance and an opposite electrode 2420 are formed over the pixelelectrode 2417, and a light-emitting element 2421 is formed in a regionwhere the layer 2419 containing a light-emitting substance is sandwichedbetween the pixel electrode 2417 and the opposite electrode 2420. Notethat the opening 2415 is located below the light-emitting element 2421.That is, since the first interlayer insulating film 2412 has the opening2415, transmittance can be increased when light emission from thelight-emitting element 2421 is extracted from the substrate side.

By using a crystalline semiconductor film for the semiconductor layer ofthe transistor included in the pixel of the invention, the scan linedriver circuit 912 and the signal line driver circuit 911 in FIG. 9 canbe easily formed over the same substrate as the pixel portion 913, forexample.

Note that the structure of the transistor using the crystallinesemiconductor film for the semiconductor layer is also not limited tothat described above, and various structures can be employed. This isalso true for a capacitor. In this embodiment mode, the materials inFIG. 17 can be used as appropriate unless stated otherwise.

The transistor described in this embodiment mode can be used as thetransistor of controlling a current value supplied to the light-emittingelement in each pixel described in Embodiment Modes 1 to 6. Therefore,variations of the current value caused by variations in thresholdvoltage of the transistor can be suppressed by operating the pixel asthe described manner in any of Embodiment Modes 1 to 6. Accordingly, acurrent in accordance with luminance data can be supplied to alight-emitting element, so that variations in luminance can besuppressed. In addition, since operation is performed with the potentialof the opposite electrode fixed, power consumption can be reduced.

Further, by applying such a pixel to the display device shown in FIG. 9,since each pixel can emit light except in its address period, a ratio ofa light-emitting period in one frame period (i.e., a duty ratio) can beextremely raised and can also be approximately 100%. Accordingly, adisplay device with few variations in luminance and a high duty ratiocan be obtained.

In addition, since the threshold voltage writing period can be set long,the threshold voltage of the transistor of controlling a current valuesupplied to the light-emitting element can be written in the capacitormore accurately. Therefore, reliability as a display device can beimproved.

Embodiment Mode 8

In this embodiment mode, an element having a structure which isdifferent from the light-emitting element described in Embodiment Mode 7is described.

A light-emitting element utilizing electroluminescence is distinguishedby whether a light-emitting material is an organic compound or aninorganic compound. In general, the former is called an organic ELelement, and the latter is called an inorganic EL element.

The inorganic EL element is classified into a dispersion type inorganicEL element and a thin-film type inorganic EL element, depending on itselement structure. The former and the latter are different in that theformer has a light-emitting layer where particles of a light-emittingmaterial are dispersed in a binder whereas the latter has alight-emitting layer formed of a thin film of a light-emitting material.However, the former and the latter have in common that electronsaccelerated by a high electric field are required. Note that, as amechanism of light emission that is obtained, there are donor-acceptorrecombination type light emission that utilizes a donor level and anacceptor level, and localized type light emission that utilizesinner-shell electron transition of a metal ion. In general, in manycases, a dispersion type inorganic EL element has donor-acceptorrecombination type light emission, and a thin-film type inorganic ELelement has localized type light emission.

The light-emitting material used in this embodiment mode includes atleast a host material and an impurity element to be a light-emissioncenter (also called a light-emitting substance). By changing an impurityelement that is contained, light emission of various colors can beobtained. As a manufacturing method of the light-emitting material,various methods such as a solid phase method and a liquid phase method(a coprecipitation method) can be used. Further, an evaporativedecomposition method, a double decomposition method, a method by heatdecomposition reaction of a precursor, a reversed micelle method, amethod in which such a method is combined with high temperature baking,a liquid phase method such as a lyophilization method, or the like canalso be used.

A solid phase method is a method in which a host material, and animpurity element or a compound containing an impurity element areweighed, mixed in a mortar, heated in an electric furnace, and baked tobe reacted, thereby containing the impurity element in the hostmaterial. The baking temperature is preferably 700° C. to 1500° C. Thisis because the solid reaction does not progress when the temperature istoo low, whereas the host material is decomposed when the temperature istoo high. The baking may be performed in a powder state; however, it ispreferable to perform the baking in a pellet state. Although the bakinghas to be performed at a comparatively high temperature, the solid phasemethod is easy; thus, the solid phase method is suitable for massproduction with high productivity.

A liquid phase method (a coprecipitation method) is a method in which ahost material or a compound containing a host material is reacted withan impurity element or a compound containing an impurity element in asolution, dried, and then baked. Particles of a light-emitting materialare distributed uniformly, and the reaction can progress even when thegrain size is small and the baking temperature is low.

As a host material used for a light-emitting material, hydrosulfide,oxide, or nitride can be used. As hydrosulfide, for example, zincsulfide (ZnS), cadmium sulfide (CdS), calcium sulfide (CaS), yttriumsulfide (Y₂S₃), gallium sulfide (Ga₂S₃), strontium sulfide (SrS), bariumsulfide (BaS), or the like can be used. As oxide, for example, zincoxide (ZnO), yttrium oxide (Y₂O₃), or the like can be used. As nitride,for example, aluminum nitride (AlN), gallium nitride (GaN), indiumnitride (InN), or the like can be used. Further, zinc selenide (ZnSe),zinc telluride (ZnTe), or the like can also be used, and a ternary mixedcrystal such as calcium sulfide-gallium (CaGa₂S₄), strontiumsulfide-gallium (SrGa₂S₄), or barium sulfide-gallium (BaGa₂S₄) may alsobe used.

As a light-emission center of localized type light emission, manganese(Mn), copper (Cu), samarium (Sm), terbium (Tb), erbium (Er), thulium(Tm), europium (Eu), cerium (Ce), praseodymium (Pr), or the like can beused. Note that a halogen element such as fluorine (F) or chlorine (Cl)may be added as charge compensation.

On the other hand, as a light-emission center of donor-acceptorrecombination type light emission, a light-emitting material containinga first impurity element which forms a donor level and a second impurityelement which forms an acceptor level can be used. As the first impurityelement, for example, fluorine (F), chlorine (Cl), aluminum (Al), or thelike can be used. As the second impurity element, for example, copper(Cu), silver (Ag), or the like can be used.

In the case of synthesizing the light-emitting material ofdonor-acceptor recombination type light emission by a solid phasemethod, a host material, the first impurity element or a compoundcontaining the first impurity element, and the second impurity elementor a compound containing the second impurity element are each measured,mixed in a mortar, heated in an electric furnace, and baked. As the hostmaterial, any of the above described host materials can be used. As thefirst impurity element or the compound containing the first impurityelement, for example, fluorine (F), chlorine (Cl), aluminum sulfate(Al₂S₃), or the like can be used. As the second impurity element or thecompound containing the second impurity element, for example, copper(Cu), silver (Ag), copper sulfide (Cu₂S), silver sulfide (Ag₂S), or thelike can be used. The baking temperature is preferably 700° C. to 1500°C. This is because the solid reaction does not progress when thetemperature is too low, whereas the host material is decomposed when thetemperature is too high. Note that although the baking may be performedin a powder state, it is preferable to perform the baking in a pelletstate.

As the impurity element in the case of utilizing solid reaction, thecompound containing the first impurity element and the second impurityelement may be combined. In this case, since the impurity element iseasily diffused and solid reaction progresses easily, a uniformlight-emitting material can be obtained. Further, since an unnecessaryimpurity element does not enter, a light-emitting material having highpurity can be obtained. As the compound containing the first impurityelement and the second impurity element, for example, copper chloride(CuCl), silver chloride (AgCl), or the like can be used.

Note that the concentration of these impurity elements may be 0.01 to 10atom % with respect to the host material, and is preferably 0.05 to 5atom %.

In the case of a thin-film type inorganic EL element, a light-emittinglayer is a layer containing the above light-emitting material, which canbe formed by a vacuum evaporation method such as a resistance heatingevaporation method or an electron beam evaporation (EB evaporation)method, a physical vapor deposition (PVD) method such as a sputteringmethod, a chemical vapor deposition (CVD) method such as an organicmetal CVD method or a hydride transport low-pressure CVD method, anatomic layer epitaxy method (ALE), or the like.

FIGS. 46A to 46C each show an example of a thin-film type inorganic ELelement that can be used as a light-emitting element. In FIGS. 46A to46C, each light-emitting element includes a first electrode 4601, alight-emitting layer 4602, and a second electrode 4603.

The light-emitting elements shown in FIGS. 46B and 46C each have astructure where an insulating layer is provided between the electrodeand the light-emitting layer of the light-emitting element of FIG. 46A.The light-emitting element shown in FIG. 46B has an insulating layer4604 between the first electrode 4601 and the light-emitting layer 4602.The light-emitting element shown in FIG. 46C includes an insulatinglayer 4604 a between the first electrode 4601 and the light-emittinglayer 4602, and an insulating layer 4604 b between the second electrode4603 and the light-emitting layer 4602. In this manner, the insulatinglayer may be provided between the light-emitting layer and one electrodeof a pair of electrodes that sandwiches the light-emitting layer, or maybe provided between the light-emitting layer and the first electrode andbetween the light-emitting layer and the second electrode. Moreover, theinsulating layer may be a single layer or a stacked layer including aplurality of layers.

In addition, although the insulating layer 4604 is provided so as to bein contact with the first electrode 4601 in FIG. 46B, the insulatinglayer 4604 may be provided so as to be in contact with the secondelectrode 4603 by reversing the order of the insulating layer and thelight-emitting layer.

In the case of a dispersion type inorganic EL element, a light-emittinglayer film where particles of a light-emitting material are dispersed ina binder is formed. When particles with desired grain sizes cannot beobtained by a manufacturing method of a light-emitting material,processing into a particle state may be performed by being crushed witha mortar or the like. The binder refers to a substance for fixing alight-emitting material in a particle state in a dispersed state to keepa shape as a light-emitting layer. The light-emitting material isuniformly dispersed and fixed in the light-emitting layer by the binder.

In the case of a dispersion type inorganic EL element, as a formingmethod of a light-emitting layer, a droplet-discharging method which canselectively form a light-emitting layer, a printing method (e.g., screenprinting or offset printing), a coating method such as a spin coatingmethod, a dipping method, a dispenser method, or the like can be used.There are no particular limitations on the film thickness of thelight-emitting layer; however, a film thickness of 10 nm to 1000 nm ispreferable. In addition, in the light-emitting layer containing alight-emitting material and a binder, a ratio of the light-emittingmaterial is preferably set to be equal to or more than 50 wt % and equalto or less than 80 wt %.

FIGS. 47A to 47C each show an example of a dispersion type inorganic ELelement that can be used as a light-emitting element. In FIG. 47A, thelight-emitting element has a stacked-layer structure of the firstelectrode 4601, a light-emitting layer 4702, and the second electrode4603, where a light-emitting material 4710 held by a binder is containedin the light-emitting layer 4702.

As the binder that can be used in this embodiment mode, an organicmaterial having insulating properties or an inorganic material can beused, or a mixed material of an organic material and an inorganicmaterial may also be used. As the organic material, a resin such as apolymer, polyethylene, polypropylene, a polystyrene-based resin, asilicone resin, an epoxy resin, or vinylidene fluoride having acomparatively high dielectric constant like a cyanoethyl cellulose-basedresin can be used. In addition, a heat-resistant high molecule such asaromatic polyamide or polybenzimidazole, or a siloxane resin may beused. A siloxane resin corresponds to a resin containing a Si—O—Si bond.Siloxane is composed of a skeleton structure formed by the bond ofsilicon (Si) and oxygen (O). As a substituent thereof, an organic groupcontaining at least hydrogen (e.g., an alkyl group or aryl group) isused. In addition, a fluoro group may be used as the substituent.Further, an organic group containing at least hydrogen and a fluorogroup may be used as the substituent. Moreover, a vinyl resin such aspolyvinyl alcohol or polyvinyl butyral, or a resin material such as aphenol resin, a novolac resin, an acrylic resin, a melamine resin, aurethane resin, an oxazole resin (polybenzoxazole) may also be used asthe organic material as well as the above-described materials. Adielectric constant can also be controlled by mixing these resins withmicroparticles having a high dielectric constant such as barium titanate(BaTiO₃) or strontium titanate (SrTiO₃) as appropriate.

As the inorganic material contained in the binder, a material selectedfrom silicon oxide (SiO_(x)), silicon nitride (SiN,), silicon containingoxygen and nitrogen, aluminum nitride (AlN), aluminum containing oxygenand nitrogen or aluminum oxide (Al₂O₃), titanium oxide (TiO₂), BaTiO₃,SrTiO₃, lead titanate (PbTiO₃), potassium niobate (KNbO₃), lead niobate(PbNbO₃), tantalum oxide (Ta₂O₅), barium tantalate (BaTa₂O₆), lithiumtantalate (LiTaO₃), yttrium oxide (Y₂O₃), zirconium oxide (ZrO₂), zincsulfide (ZnS) and other substances containing an inorganic material canbe used. By mixing the organic material with an inorganic materialhaving a high dielectric constant (by adding or the like), a dielectricconstant of a light-emitting layer including a light-emitting materialand a binder can be further increased.

In a manufacturing process, the light-emitting material is dispersed ina solution containing a binder. As a solvent of the solution containinga binder that can be used in this embodiment mode, it is preferable toselect such a solvent that dissolves a binder material and that can makea solution with the viscosity of which is appropriate for a method forforming the light-emitting layer (various wet processes) and a desiredfilm thickness. An organic solvent or the like can be used and, forexample, when a siloxane resin is used as the binder, propyleneglycolmonomethyl ether, propylene glycolmonomethyl ether acetate (alsocalled PGMEA), 3-methoxy-3-methyl-1-butanol (also called MMB), or thelike can be used.

The light-emitting elements shown in FIGS. 47B and 47C each have astructure where an insulating layer is provided between the electrodeand the light-emitting layer of the light-emitting element of FIG. 47A.The light-emitting element shown in FIG. 47B has the insulating layer4604 between the first electrode 4601 and the light-emitting layer 4702.The light-emitting element shown in FIG. 47C has the insulating layer4604 a between the first electrode 4601 and the light-emitting layer4702, and the insulating layer 4604 b between the second electrode 4603and the light-emitting layer 4702. In this manner, the insulating layermay be provided between the light-emitting layer and one electrode of apair of electrodes that sandwiches the light-emitting layer, or may beprovided between the light-emitting layer and the first electrode 4601and between the light-emitting layer and the second electrode 4603.Moreover, the insulating layer may be a single layer or a stacked layerincluding a plurality of layers.

In addition, although the insulating layer 4604 is provided so as to bein contact with the first electrode 4601 in FIG. 47B, the insulatinglayer 4604 may be provided so as to be in contact with the secondelectrode 4603 by reversing the order of the insulating layer and thelight-emitting layer.

Although the insulating layers 4604, 4604 a and 4604 b in FIGS. 46B,46C, 47B, and 47C are not particularly limited, such insulating layerspreferably have high dielectric strength and dense film qualities, andmore preferably have high dielectric constants. For example, siliconoxide (SiO₂), yttrium oxide (Y₂O₃), titanium oxide (TiO₂), aluminumoxide (Al₂O₃), hafnium oxide (HfO₂), tantalum oxide (Ta₂O₅), bariumtitanate (BaTiO₃), strontium titanate (SrTiO₃), lead titanate (PbTiO₃),silicon nitride (Si₃N₄), zirconium oxide (ZrO₂), or the like, or a mixedfilm or a staked-layer film of two kinds or more thereof can be used.These insulating films can be formed by sputtering, evaporation, CVD, orthe like. In addition, the insulating layers may be formed by dispersingparticles of these insulating materials in the binder. The bindermaterial may be formed with the same material and by the same method asthe binder contained in the light-emitting layer. A film thickness ofsuch an insulating layer is not particularly limited, and a filmthickness of 10 nm to 1000 nm is preferable.

For the first electrode 4601 and the second electrode 4603, a metal, analloy, a conductive compound, a mixture thereof, or the like can beused. For example, each material can be selected as appropriate from thematerials used for the pixel electrode 1801 and the opposite electrode1802 described in Embodiment Mode 7.

Note that the light-emitting element described in this embodiment modecan emits light when a voltage is applied between the pair of electrodeswhich sandwiches the light-emitting layer, namely to the first electrode4601 and the second electrode 4603.

An inorganic EL element thus obtained can be used as the light-emittingelement in Embodiment Mode 7, and can be combined freely with the otherembodiment modes.

Embodiment Mode 9

In this embodiment mode, one mode of a display device of the inventionis described with reference to FIGS. 25A and 25B.

FIG. 25A is a top plan view showing a display device, and FIG. 25B is anA-A′ line cross sectional view (cross sectional view taken along a lineA-A′) of FIG. 25A. The display device includes a signal line drivercircuit 2501, a pixel portion 2502, a first scan line driver circuit2503, and a second scan line driver circuit 2506 over a substrate 2510which are indicated by dotted lines in the drawing. The display devicealso includes a sealing substrate 2504 and a sealant 2505, and an insideportion of the display device surrounded by them is a space 2507.

Note that a wiring 2508 is a wiring for transmitting signals to beinputted to the first scan line driver circuit 2503, the second scanline driver circuit 2506, and the signal line driver circuit 2501 andreceives a video signal, a clock signal, a start signal, and the likethrough an FPC (Flexible Printed Circuit) 2509 that serves as anexternal input terminal. IC chips (semiconductor chips provided with amemory circuit, a buffer circuit, and the like) 2518 and 2519 aremounted on a connection portion of the FPC 2509 and the display deviceby COG (Chip On Glass) or the like. Note that although only the FPC isshown here, a printed wiring board (PWB) may also be attached to theFPC. The display device of the invention includes not only a main bodyof a display device but also a display device with an FPC or a PWBattached thereto. In addition, it also includes a display device onwhich an IC chip or the like is mounted.

A cross-sectional structure is described with reference to FIG. 25B.Although the pixel portion 2502 and its peripheral driver circuits (thefirst scan line driver circuit 2503, the second scan line driver circuit2506, and the signal line driver circuit 2501) are formed over thesubstrate 2510, only the signal line driver circuit 2501 and the pixelportion 2502 are shown here.

Note that the signal line driver circuit 2501 includes transistors withthe same conductivity type such as N-channel transistors 2520 and 2521.It is needless to say that only P-channel transistors may be used or aCMOS circuit may be formed using both an N-channel transistor and aP-channel transistor. Although this embodiment mode describes a displaypanel in which the peripheral driver circuits are formed over the samesubstrate as the pixel portion, the invention is not limited to this.All or part of the peripheral driver circuits may be formed on an ICchip or the like and mounted by COG or the like.

The pixel described in any of Embodiment Modes 1 to 6 is used for thepixel portion 2502. Note that a transistor 2511 which functions as aswitch, a transistor 2512 which controls a current value supplied to alight-emitting element, and a light-emitting element 2528 are shown inFIG. 25B. Note that a first electrode of the transistor 2512 isconnected to a pixel electrode 2513 of the light-emitting element 2528.In addition, an insulator 2514 is formed so as to cover an end portionof the pixel electrode 2513. Here, the insulator 2514 is formed using apositive photosensitive acrylic resin film.

The insulator 2514 is formed to have a curved surface with a curvatureat an upper end portion or a lower end portion thereof in order toobtain excellent coverage. For example, in the case of using positivephotosensitive acrylic as a material of the insulator 2514, theinsulator 2514 is preferably formed to have a curved surface with acurvature radius (0.2 μm to 3 μm) only at the upper end portion. Eithera negative resist which becomes insoluble in an etchant by lightirradiation or a positive resist which becomes soluble in an etchant bylight irradiation can be used as the insulator 2514.

A layer 2516 containing a light-emitting substance and an oppositeelectrode 2517 are formed over the pixel electrode 2513. As for thelayer 2516 containing a light-emitting substance, as long as at least alight-emitting layer is provided, there is no particular limitation onlayers other than the light-emitting layer and they can be selected asappropriate.

By attaching the sealing substrate 2504 to the substrate 2510 using thesealant 2505, a structure is obtained in which the light-emittingelement 2528 is provided in the space 2507 surrounded by the substrate2510, the sealing substrate 2504, and the sealant 2505. Note that thereis also a case where the space 2507 is filled with the sealant 2505other than an inert gas (e.g., nitrogen or argon).

Note that an epoxy-based resin is preferably used as the sealant 2505.The material preferably allows as little moisture and oxygen as possibleto penetrate. As the sealing substrate 2504, a plastic substrate formedof FRP (Fiberglass-Reinforced Plastics), PVF (polyvinyl fluoride),Mylar, polyester, acrylic, or the like can be used as well as a glasssubstrate or a quartz substrate.

Variations in luminance among pixels or fluctuation in luminance of apixel over time can be suppressed by using and operating any of thepixels described in Embodiment Modes 1 to 6 in the pixel portion 2502,and thus a high quality display device with a higher duty ratio can beobtained. In addition, power consumption can be reduced in the inventionbecause operation is performed with the potential of the oppositeelectrode fixed.

By forming the signal line driver circuit 2501, the pixel portion 2502,the first scan line driver circuit 2503, and the second scan line drivercircuit 2506 over the same substrate as shown in FIGS. 25A and 25B, costof the display device can be reduced. In this case, a manufacturingprocess can be simplified by using transistors with the sameconductivity type for the signal line driver circuit 2501, the pixelportion 2502, the first scan line driver circuit 2503, and the secondscan line driver circuit 2506; accordingly, further cost reduction canbe achieved.

In this manner, the display device of the invention can be obtained.Note that the above-described structure is one example and a structureof the display device of the invention is not limited to this.

Note that as the structure of the display device, there may be astructure in which a signal line driver circuit 2601 is formed on an ICchip and the IC chip is mounted on a display device by COG or the likeas shown in FIG. 26A. Note that a substrate 2600, a pixel portion 2602,a first scan line driver circuit 2603, a second scan line driver circuit2604, an FPC 2605, an IC chip 2606, an IC chip 2607, a sealing substrate2608, and a sealant 2609 of FIG. 26A correspond to the substrate 2510,the pixel portion 2502, the first scan line driver circuit 2503, thesecond scan line driver circuit 2506, the FPC 2509, the IC chip 2518,the IC chip 2519, the sealing substrate 2504, and the sealant 2505 inFIG. 25A, respectively.

That is, only a signal line driver circuit of which high speed operationis required is formed on an IC chip by using a CMOS or the like toreduce power consumption. In addition, higher-speed operation and lowerpower consumption can be achieved by using a semiconductor chip made ofa silicon wafer or the like as the IC chip.

Note that cost reduction can be achieved by forming the first scan linedriver circuit 2603 and the second scan line driver circuit 2604 overthe same substrate as the pixel portion 2602. A further cost reductioncan be achieved by forming the first scan line driver circuit 2603, thesecond scan line driver circuit 2604, and the pixel portion 2602 usingtransistors with the same conductivity type. At this time, decrease inoutput potential can be prevented by using boot trap circuits for thefirst scan line driver circuit 2603 and the second scan line drivercircuit 2604. In addition, in the case of using amorphous silicon forsemiconductor layers of transistors included in the first scan linedriver circuit 2603 and the second scan line driver circuit 2604, sincethe threshold voltage of each transistor fluctuates due todeterioration, it is preferable to provide a function to correct thefluctuation.

Variations in luminance among pixels or fluctuation in luminance of apixel over time can be suppressed by using and operating any of thepixels described in Embodiment Modes 1 to 6 in the pixel portion 2602,and thus a high quality display device with a higher duty ratio can beobtained. In addition, power consumption can be reduced in the inventionbecause operation is performed with the potential of the oppositeelectrode fixed. In addition, a substrate area can be used efficientlyby mounting an IC chip provided with a functional circuit (a memory or abuffer) on a connection portion of the FPC 2605 and the substrate 2600.

Further, a structure may also be employed in which a signal line drivercircuit 2611, a first scan line driver circuit 2613, and a second scanline driver circuit 2614 corresponding to the signal line driver circuit2501, the first scan line driver circuit 2503, and the second scan linedriver circuit 2506 of FIG. 25A are formed on IC chips and the IC chipsare mounted on a display device by COG or the like as shown in FIG. 26B.Note that a substrate 2610, a pixel portion 2612, an FPC 2615, an ICchip 2616, an IC chip 2617, a sealing substrate 2618, and a sealant 2619of FIG. 26B correspond to the substrate 2510, the pixel portion 2502,the FPC 2509, the IC chip 2518, the IC chip 2519, the sealing substrate2504, and the sealant 2505 of FIG. 25A, respectively.

Further, cost reduction can be achieved by using a non-crystallinesemiconductor film, e.g., an amorphous silicon (a-Si:H) film for asemiconductor layer of a transistor of the pixel portion 2612. Further,a large-sized display panel can also be manufactured.

Further, the first scan line driver circuit, the second scan line drivercircuit, and the signal line driver circuit are not necessarily providedin a row direction and a column direction of pixels. For example, asshown in FIG. 27A, a peripheral driver circuit 2701 formed on an IC chipmay have functions of the first scan line driver circuit 2613, thesecond scan line driver circuit 2614, and the signal line driver circuit2611 shown in FIG. 26B. Note that a substrate 2700, a pixel portion2702, an FPC 2704, an IC chip 2705, an IC chip 2706, a sealing substrate2707, and a sealant 2708 of FIG. 27A correspond to the substrate 2510,the pixel portion 2502, the FPC 2509, the IC chip 2518, the IC chip2519, the sealing substrate 2504, and the sealant 2505 of FIG. 25A,respectively.

Note that a schematic diagram illustrating the connection of wirings ofthe display device of FIG. 27A is shown in FIG. 27B. A substrate 2710, aperipheral driver circuit 2711, a pixel portion 2712, an FPC 2713, andan FPC 2714 are shown in FIG. 27B.

The FPC 2713 and the FPC 2714 input signals and power supply potentialsfrom outside to the peripheral driver circuit 2711. Then, an output fromthe peripheral driver circuit 2711 is inputted to wirings in row andcolumn directions connected to pixels included in the pixel portion2712.

Further, in the case of using a white light-emitting element as thelight-emitting element, full color display can be realized by providingthe sealing substrate with color filters. The invention can also beapplied to such a display device. FIG. 28 shows one example of a partialsectional view of a pixel portion.

As shown in FIG. 28, a base film 2802 is formed over a substrate 2800; atransistor 2801 which controls a current value supplied to alight-emitting element is formed thereover; and a pixel electrode 2803is formed in contact with a first electrode of the transistor 2801. Alayer 2804 containing a light-emitting substance and an oppositeelectrode 2805 are formed thereover.

Note that a portion where the layer 2804 containing a light-emittingsubstance is sandwiched between the pixel electrode 2803 and theopposite electrode 2805 serves as the light-emitting element. Note thatwhite light is emitted in FIG. 28. A red color filter 2806R, a greencolor filter 28066, and a blue color filter 2806B are provided above thelight-emitting elements to achieve full-color display. In addition, ablack matrix (also referred to as a BM) 2807 is provided to separatethese color filters.

The display device of this embodiment mode can be combined with thestructure described in Embodiment Mode 7 or 8 as appropriate as well asthose in Embodiment Modes 1 to 6. In addition, the structure of thedisplay device is not limited to that described above, and the inventioncan also be applied to a display device having another structure.

Embodiment Mode 10

The display device of the invention can be applied to various electronicdevices. Specifically, it can be applied to a display portion of anelectronic device. Note that examples of the electronic devices are asfollows: a camera such as a video camera or a digital camera, a goggletype display, a navigation system, an audio-reproducing device (e.g.,car audio or an audio component), a computer, a game machine, a portableinformation terminal (e.g., a mobile computer, a mobile phone, a mobilegame machine, or an electronic book), an image-reproducing device havinga recording medium (specifically, a device for reproducing a content ofa recording medium such as a digital versatile disc (DVD) and having adisplay for displaying a reproduced image), and the like.

FIG. 33A shows a display which includes a housing 3301, a support 3302,a display portion 3303, a speaker portion 3304, a video input terminal3305, and the like.

Note that the pixel described in any of Embodiment Modes 1 to 6 is usedfor the display portion 3303. By employing the invention, variations inluminance among pixels or fluctuation in luminance of a pixel over timecan be suppressed and a display including a high quality display portionwith a higher duty ratio can be obtained. Further, power consumption canbe reduced in the invention because operation is performed with thepotential of an opposite electrode fixed. Note that the display includesin its category all display devices used for displaying information,e.g., for a personal computer, for TV broadcast reception, or foradvertisement display.

Note that while needs for increase in display size have been increasing,an increase in price associated with the increase in display size hasbecome an issue. Therefore, it is an issue to reduce manufacturing costas much as possible and set the price of a high-quality product as lowas possible.

Since the pixel of the invention can be manufactured using transistorswith the same conductivity type, the number of steps can be reduced andmanufacturing cost can be reduced. Moreover, a process can be simplifiedand further cost reduction can be achieved by using a non-crystallinesemiconductor film, e.g., an amorphous silicon (a-Si:H) film for asemiconductor layer of each transistor included in the pixel. In thiscase, a driver circuit at the periphery of a pixel portion is preferablyformed on an IC chip and the IC chip is mounted on a display panel byCOG (Chip On Glass) or the like. Note that a signal line driver circuitwith high operation speed may be formed on an IC chip, and a scan linedriver circuit with relatively low operation speed may be formed using acircuit including transistors with the same conductivity type over thesame substrate as the pixel portion.

FIG. 33B shows a camera which includes a main body 3311, a displayportion 3312, an image receiving portion 3313, operation keys 3314, anexternal connection port 3315, a shutter 3316, and the like.

Note that the pixel described in any of Embodiment Modes 1 to 6 is usedfor the display portion 3312. By employing the invention, variations inluminance among pixels or fluctuation in luminance of a pixel over timecan be suppressed, and a camera including a high quality display portionwith a higher duty ratio can be obtained. Further, power consumption canbe reduced in the invention because operation is performed with thepotential of the opposite electrode fixed.

In addition, competitive manufacturing of a digital camera or the likehas been intensified along with improvement in performance. Therefore,it is important to set the price of a high-performance product as low aspossible.

Since the pixel of the invention can be manufactured using transistorswith the same conductivity type, the number of steps can be reduced andmanufacturing cost can be reduced. Further, a process can be simplifiedand further cost reduction can be achieved by using a non-crystallinesemiconductor film, e.g., an amorphous silicon (a-Si:H) film for asemiconductor layer of each transistor included in the pixel. In thiscase, a driver circuit at the periphery of a pixel portion is preferablyformed on an IC chip and the IC chip is mounted on a display panel byCOG or the like. Note that a signal line driver circuit with highoperation speed may be formed on an IC chip, and a scan line drivercircuit with relatively low operation speed may be formed using acircuit including transistors with the same conductivity type over thesame substrate as the pixel portion.

FIG. 33C shows a computer which includes a main body 3321, a chassis3322, a display portion 3323, a keyboard 3324, an external connectionport 3325, a pointing device 3326, and the like. Note that the pixeldescribed in any of Embodiment Modes 1 to 6 is used for the displayportion 3323. By employing the invention, variations in luminance amongpixels or fluctuation in luminance of a pixel over time can besuppressed and a computer including a high quality display portion witha higher duty ratio can be obtained. Further, power consumption can bereduced in the invention because operation is performed with thepotential of the opposite electrode fixed. Further, cost reduction canbe achieved by using transistors with the same conductivity type, astransistors included in the pixel portion or using a non-crystallinesemiconductor film for semiconductor layers of the transistors.

FIG. 33D shows a mobile computer which includes a main body 3331, adisplay portion 3332, a switch 3333, operation keys 3334, an infraredport 3335, and the like. Note that the pixel described in any ofEmbodiment Modes 1 to 6 is used for the display portion 3332. Byemploying the invention, variations in luminance among pixels orfluctuation in luminance of a pixel over time can be suppressed and amobile computer including a high quality display portion with a higherduty ratio can be obtained. Further, power consumption can be reduced inthe invention because operation is performed with the potential of theopposite electrode fixed. Further, cost reduction can be achieved byusing transistors with the same conductivity type, as transistorsincluded in the pixel portion or using a non-crystalline semiconductorfilm for semiconductor layers of the transistors.

FIG. 33E shows a portable image reproducing device provided with arecording medium (specifically, a DVD player) which includes a main body3341, a chassis 3342, a display portion A 3343, a display portion B3344, a recording medium (e.g., DVD) reading portion 3345, operationkeys 3346, a speaker portion 3347, and the like. The display portion A3343 mainly displays image information, and the display portion B 3344mainly displays character information. Note that the pixel described inany of Embodiment Modes 1 to 6 is used for the display portion A 3343and the display portion B 3344. By employing the invention, variationsin luminance among pixels or fluctuation in luminance of a pixel overtime can be suppressed and an image reproducing device including a highquality display portion with a higher duty ratio can be obtained.Further, power consumption can be reduced in the invention becauseoperation is performed with the potential of the opposite electrodefixed. Further, cost reduction can be achieved by using transistors withthe same conductivity type, as transistors included in the pixel portionor using a non-crystalline semiconductor film for semiconductor layersof the transistors.

FIG. 33F shows a goggle type display which includes a main body 3351, adisplay portion 3352, an arm portion 3353, and the like. Note that thepixel described in any of Embodiment Modes 1 to 6 is used for thedisplay portion 3352. By employing the invention, variations inluminance among pixels or fluctuation in luminance of a pixel over timecan be suppressed and a goggle type display including a high qualitydisplay portion with a higher duty ratio can be obtained. Further, powerconsumption can be reduced in the invention because operation isperformed with the potential of the opposite electrode fixed. Further,cost reduction can be achieved by using transistors with the sameconductivity type, as transistors included in the pixel portion or usinga non-crystalline semiconductor film for semiconductor layers of thetransistors.

FIG. 33G shows a video camera which includes a main body 3361, a displayportion 3362, a chassis 3363, an external connection port 3364, a remotecontrol receiving portion 3365, an image receiving portion 3366, abattery 3367, an audio input portion 3368, operation keys 3369, an eyepiece portion 3360, and the like. Note that the pixel described in anyof Embodiment Modes 1 to 6 is used for the display portion 3362. Byemploying the invention, variations in luminance among pixels orfluctuation in luminance of a pixel over time can be suppressed and avideo camera including a high quality display portion with a higher dutyratio can be obtained. Further, power consumption can be reduced in theinvention because operation is performed with the potential of theopposite electrode fixed. Further, cost reduction can be achieved byusing transistors with the same conductivity type, as transistorsincluded in the pixel portion or using a non-crystalline semiconductorfilm for semiconductor layers of the transistors.

FIG. 33H shows a mobile phone which includes a main body 3371, a chassis3372, a display portion 3373, an audio input portion 3374, an audiooutput portion 3375, operation keys 3376, an external connection port3377, an antenna 3378, and the like. Note that the pixel described inany of Embodiment Modes 1 to 6 is used for the display portion 3373. Byemploying the invention, variations in luminance among pixels orfluctuation in luminance of a pixel over time can be suppressed and amobile phone including a high quality display portion with a higher dutyratio can be obtained. Further, power consumption can be reduced in theinvention because operation is performed with the potential of theopposite electrode fixed. Further, cost reduction can be achieved byusing transistors with the same conductivity type, as transistorsincluded in the pixel portion or using a non-crystalline semiconductorfilm for semiconductor layers of the transistors.

As described above, the invention can be applied to any electronicdevice.

Embodiment Mode 11

In this embodiment mode, a structure example of a mobile phone includingthe display device of the invention in a display portion is describedwith reference to FIG. 34.

A display panel 3410 is incorporated in a housing 3400 so as to bedetachable. The shape and size of the housing 3400 can be changed asappropriate in accordance with the size of the display panel 3410. Thehousing 3400 to which the display panel 3410 is fixed is fitted in aprinted circuit board 3401 and assembled as a module.

The display panel 3410 is connected to the printed circuit board 3401through an FPC 3411. The printed circuit board 3401 is provided with aspeaker 3402, a microphone 3403, a transmitting/receiving circuit 3404,and a signal processing circuit 3405 including a CPU, a controller, andthe like. Such a module, an input unit 3406, and a buttery 3407 arecombined and stored in a chassis 3409 and a chassis 3412. Note that apixel portion of the display panel 3410 is arranged so as to be seenfrom a window formed in the chassis 3412.

In the display panel 3410, the pixel portion and a part of peripheraldriver circuits (a driver circuit having a low operation frequency amonga plurality of driver circuits) may be formed using transistors over asubstrate, and another part of the peripheral driver circuits (a drivercircuit having a high operation frequency among the plurality of drivercircuits) may be formed on an IC chip. The IC chip may be mounted on thedisplay panel 3410 by COG (Chip On Glass). The IC chip may alternativelybe connected to a glass substrate by using TAB (Tape Automated Bonding)or a printed circuit board. Further, all of the peripheral drivercircuits may be formed on an IC chip and the IC chip may be mounted onthe display panel by COG or the like.

Note that the pixel described in any of Embodiment Modes 1 to 6 is usedfor the pixel portion. By employing the invention, variations inluminance among pixels or fluctuation in luminance of a pixel over timecan be suppressed and the display panel 3410 including a high qualitydisplay portion with a higher duty ratio can be obtained. Further, powerconsumption can be reduced in the invention because operation isperformed with the potential of the opposite electrode fixed. Further,cost reduction can be achieved by using transistors with the sameconductivity type, as transistors included in the pixel portion or usinga non-crystalline semiconductor film for semiconductor layers of thetransistors.

The structure of the mobile phone described in this embodiment mode isjust one example, and the display device of the invention can be appliednot only to the mobile phone having the above-described structure butalso to mobile phones having various kinds of structures.

Embodiment Mode 12

In this embodiment mode, an EL module obtained by combining a displaypanel and a circuit board is described with reference to FIGS. 35 and36.

As shown in FIG. 35, a display panel 3501 includes a pixel portion 3503,a scan line driver circuit 3504, and a signal line driver circuit 3505.Over a circuit board 3502, for example, a control circuit 3506, a signaldividing circuit 3507, and the like are formed. Note that the displaypanel 3501 and the circuit board 3502 are connected to each other by aconnection wiring 3508. As the connection wiring 3508, an FPC or thelike can be used.

In the display panel 3501, the pixel portion and a part of peripheraldriver circuits (a driver circuit having a low operation frequency amonga plurality of driver circuits) may be formed using transistors over asubstrate, and another part of the peripheral driver circuits (a drivercircuit having a high operation frequency among the plurality of drivercircuits) may be formed on an IC chip. The IC chip may be mounted on thedisplay panel 3501 by COG (Chip On Glass). The IC chip may alternativelybe connected to a glass substrate by using TAB (Tape Automated Bonding)or a printed circuit board. Further, all of the peripheral drivercircuits may be formed on an IC chip and the IC chip may be mounted onthe display panel by COG or the like.

Note that the pixel described in any of Embodiment Modes 1 to 6 is usedfor the pixel portion. By employing the invention, variations inluminance among pixels or fluctuation in luminance of a pixel over timecan be suppressed and the high quality display panel 3501 with a higherduty ratio can be obtained. Further, power consumption can be reduced inthe invention because operation is performed with the potential of anopposite electrode fixed. Further, cost reduction can be achieved byusing transistors with the same conductivity type, as transistorsincluded in the pixel portion or using a non-crystalline semiconductorfilm for semiconductor layers of the transistors.

An EL TV receiver can be completed with such an EL module. FIG. 36 is ablock diagram showing a main structure of an EL TV receiver. A tuner3601 receives a video signal and an audio signal. The video signal isprocessed by a video signal amplifier circuit 3602, a video signalprocessing circuit 3603 for converting a signal output from the videosignal amplifier circuit 3602 into a color signal corresponding to eachcolor of red, green, and blue, and a control circuit 3506 for convertingthe video signal into a signal which meets input specifications of adriver circuit. The control circuit 3506 outputs signals to a scan lineside and a signal line side. In the case of performing a digital drive,a structure can be employed in which the signal dividing circuit 3507 isprovided on the signal line side to supply an input digital signaldivided into m pieces.

The audio signal among the signals received by the tuner 3601 istransmitted to an audio signal amplifier circuit 3604, and an output ofthe audio signal amplifier circuit 3604 is supplied to a speaker 3606through an audio signal processing circuit 3605. A control circuit 3607receives control information of a receiving station (receptionfrequency) or sound volume from an input portion 3608, and transmitssignals to the tuner 3601 and the audio signal processing circuit 3605.

By incorporating the EL module in FIG. 35 into the chassis 3301 of FIG.33A described in Embodiment Mode 10, a TV receiver can be completed.

Needless to say, the invention is not limited to the TV receiver, andcan be applied to various uses particularly as a large-sized displaymedium such as an information display board at a train station, anairport, or the like, or an advertisement display board on the street,as well as a monitor of a personal computer.

This application is based on Japanese Patent Application Serial No.2006-104191 filed in Japan Patent Office on 5 Apr. 2006, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a firsttransistor, a second transistor, and a third transistor, and a fourthtransistor; a capacitor; a pixel electrode; a power supply line; a gateline; and a signal line, wherein one of a source and a drain of thefirst transistor is electrically connected to one of a source and adrain of the second transistor, a first electrode of the capacitor, andthe pixel electrode, wherein a gate of the first transistor iselectrically connected to one of a source and a drain of the thirdtransistor, and a second electrode of the capacitor, wherein the otherof the source and the drain of the first transistor is electricallyconnected to one of a source and a drain of the fourth transistor,wherein the other of the source and the drain of the fourth transistoris electrically connected to the power supply line, wherein a gate ofthe third transistor is electrically connected to the gate line, whereinthe signal line is electrically connected to the other of the source andthe drain of the third transistor, wherein a channel forming region ofthe first transistor comprises a first part and a second part, wherein adirection of a channel length of the first part crosses to a directionof a third part of the signal line, wherein a direction of a channellength of the second part crosses to a direction of a fourth part of thegate line, wherein a direction of a channel width of the first partcrosses to a direction of a fifth part of the gate line, and wherein adirection of a channel width of the second part crosses to a directionof sixth part of the signal line.
 2. The semiconductor device accordingto claim 1, wherein a potential supplied from the power supply linecomprises a first potential and a second potential, and wherein thesecond potential is larger than the first potential.
 3. Thesemiconductor device according to claim 1, further comprising: a fifthtransistor, wherein one of a source and a drain of the fifth transistoris electrically connected to the gate of the first transistor, andwherein the other of the source and the drain of the fifth transistor iselectrically connected to the power supply line.
 4. The semiconductordevice comprising: a first transistor, a second transistor, a thirdtransistor, and a fourth transistor; a capacitor; a pixel electrode; apower supply line; a gate line; and a signal line, wherein one of asource and a drain of the first transistor is electrically connected toone of a source and a drain of the second transistor, a first electrodeof the capacitor, and the pixel electrode, wherein a gate of the firsttransistor is electrically connected to one of a source and a drain ofthe third transistor, and a second electrode of the capacitor, whereinthe other of the source and the drain of the first transistor iselectrically connected to one of a source and a drain of the fourthtransistor, wherein the other of the source and the drain of the fourthtransistor is electrically connected to the power supply line, wherein agate of the third transistor is electrically connected to the gate line,wherein the signal line is electrically connected to the other of thesource and the drain of the third transistor, wherein a channel formingregion of the first transistor comprises a first part and a second part,and wherein a direction of a channel length of the first part crosses toa direction of a channel length of the second part.
 5. The semiconductordevice according to claim 4, wherein a potential supplied from the powersupply line comprises a first potential and a second potential, andwherein the second potential is larger than the first potential.
 6. Thesemiconductor device according to claim 4, further comprising: a fifthtransistor, wherein one of a source and a drain of the fifth transistoris electrically connected to the gate of the first transistor, andwherein the other of the source and the drain of the fifth transistor iselectrically connected to the power supply line.
 7. A semiconductordevice comprising: a first transistor, a second transistor, a thirdtransistor, and a fourth transistor; a capacitor; a pixel electrode; apower supply line; a gate line; and a signal line, wherein one of asource and a drain of the first transistor is electrically connected toone of a source and a drain of the second transistor, a first electrodeof the capacitor, and the pixel electrode, wherein a gate of the firsttransistor is electrically connected to one of a source and a drain ofthe third transistor and a second electrode of the capacitor, whereinthe other of the source and the drain of the first transistor iselectrically connected to one of a source and a drain of the fourthtransistor, wherein the other of the source and the drain of the fourthtransistor is electrically connected to the power supply line, wherein agate of the third transistor is electrically connected to the gate line,wherein the signal line is electrically connected to the other of thesource and the drain of the third transistor, wherein a channel formingregion of the first transistor comprises a first part and a second part,wherein the first part is parallel to a third part of the signal line,and wherein the second part is parallel to a fourth part of the gateline.
 8. The semiconductor device according to claim 7, wherein apotential supplied from the power supply line comprises a firstpotential and a second potential, and wherein the second potential islarger than the first potential.
 9. A semiconductor device comprising: afirst transistor, a second transistor, a third transistor, and a fourthtransistor; a capacitor; a pixel electrode; a power supply line; a firstgate line; a second gate line; a third gate line; and a signal line,wherein one of a source and a drain of the first transistor iselectrically connected to one of a source and a drain of the secondtransistor, a first electrode of the capacitor, and the pixel electrode,wherein a gate of the first transistor is electrically connected to oneof a source and a drain of the third transistor and a second electrodeof the capacitor, wherein the other of the source and the drain of thefirst transistor is electrically connected to one of a source and adrain of the fourth transistor, wherein the other of the source and thedrain of the fourth transistor is electrically connected to the powersupply line, wherein a gate of the second transistor is electricallyconnected to the first gate line, wherein a gate of the third transistoris electrically connected to the second gate line, wherein a gate of thefourth transistor is electrically connected to the third gate line,wherein a gate of the third transistor is electrically connected to thegate line, wherein the signal line is electrically connected to theother of the source and the drain of the third transistor, wherein achannel forming region of the first transistor comprises a first partand a second part, wherein the first part is parallel to a third part ofthe signal line, and wherein the second part is parallel to at least oneof a fourth part of the first gate line, a fifth part of the second gateline, and a sixth part of the third gate line.
 10. The semiconductordevice according to claim 9, wherein a potential supplied from the powersupply line comprises a first potential and a second potential, andwherein the second potential is larger than the first potential.
 11. Thesemiconductor device according to claim 7, wherein the one of the sourceand the drain of the first transistor surrounds the other of the sourceand the drain of the first transistor.
 12. The semiconductor deviceaccording to claim 7, further comprising: a fifth transistor, whereinone of a source and a drain of the fifth transistor is electricallyconnected to the gate of the first transistor, and wherein the other ofthe source and the drain of the fifth transistor is electricallyconnected to the power supply line.
 13. The semiconductor deviceaccording to claim 9, wherein the one of the source and the drain of thefirst transistor surrounds the other of the source and the drain of thefirst transistor.
 14. The semiconductor device according to claim 9,further comprising: a fifth transistor; and a fourth gate line, whereinone of a source and a drain of the fifth transistor is electricallyconnected to the gate of the first transistor, wherein the other of thesource and the drain of the fifth transistor is electrically connectedto the power supply line, and wherein the gate of the fifth transistoris electrically connected to the fourth gate line.